View source for Spark-gap transmitter

{{Short description|Type of radio transmitter}}[[File:Morseapparat-2.JPG|thumb|upright=1.7|Low-power inductively coupled spark-gap transmitter on display in Electric Museum, [[Frastanz]], Austria. The spark gap is inside the box with the transparent cover at top center.]]

A '''spark-gap transmitter''' is an obsolete type of [[transmitter|radio transmitter]] which generates [[radio wave]]s by means of an [[electric spark]].<ref name="Hempstead">"Radio Transmitters, Early" in {{cite book
| last1= Hempstead
| first1= Colin
| last2= Worthington
| first2= William
| title= Encyclopedia of 20th-Century Technology
| publisher= Routledge
| pages= 649–650
| date= 2005
| url= https://books.google.com/books?id=2ZCNAgAAQBAJ&q=encyclopedia+%22spark+transmitter%22&pg=PA650
| isbn= 978-1135455514
}}</ref><ref name="Morris">{{cite book
| last1= Morris
| first1= Christopher G.
| title= Academic Press Dictionary of Science and Technology
| publisher= Gulf Professional Publishing
| date= 1992
| pages= 2045
| url= https://books.google.com/books?id=nauWlPTBcjIC&q=%22spark+transmitter%22&pg=PA2045
| isbn= 978-0122004001
}}</ref> Spark-gap transmitters were the first type of radio transmitter, and were the main type used during the [[wireless telegraphy]] or "spark" era, the first three decades of [[radio]], from 1887 to the end of World War I.<ref name="Champness">{{cite journal
| last1= Champness
| first1= Rodney
| title= The spark era - the beginning of radio
| journal= Silicon Chip Online
| pages= 92–97
| date= April 2010
| url= http://www.siliconchip.com.au/Issue/2010/April/The+spark+era%3A+the+beginning+of+radio
| access-date= 14 March 2018}}</ref><ref name="Terman">{{cite book |last=Terman |first=Frederick Emmons |title=Radio Engineering |edition=2nd |url=https://archive.org/stream/RadioEngineering/Terman-RadioEngineering#page/n19/mode/2up |access-date=September 14, 2015 |year=1937 |publisher=McGraw-Hill Book Co. |location=New York |pages=6–9}}</ref> German physicist [[Heinrich Hertz]] built the first experimental spark-gap transmitters in 1887, with which he proved the existence of radio waves and studied their properties.

A fundamental limitation of spark-gap transmitters is that they generate a series of brief transient pulses of radio waves called [[Damped wave (radio transmission)|damped wave]]s; they are unable to produce the [[continuous wave]]s used to carry [[audio signal|audio]] (sound) in modern [[AM broadcasting|AM]] or [[FM broadcasting|FM]] radio transmission. So spark-gap transmitters could not transmit audio, and instead transmitted information by [[radiotelegraphy]]; the operator switched the transmitter on and off with a [[telegraph key]], creating pulses of radio waves to spell out text messages in [[Morse code]].

The first practical spark gap transmitters and receivers for radiotelegraphy communication were developed by [[Guglielmo Marconi]] around 1896. One of the first uses for spark-gap transmitters was on ships, to communicate with shore and broadcast a distress call if the ship was sinking. They played a crucial role in maritime rescues such as the 1912 [[Titanic|RMS ''Titanic'']] disaster. After World War I, [[Vacuum tube oscillator|vacuum tube transmitters]]  were developed, which were less expensive and produced continuous waves which had a greater range, produced less interference, and could also carry audio, making spark transmitters obsolete by 1920. The radio signals produced by spark-gap transmitters are electrically "noisy"; they have a wide [[bandwidth (signal processing)|bandwidth]], creating [[radio frequency interference]] (RFI) that can disrupt other radio transmissions. This type of radio emission has been prohibited by international law since 1934.<ref name="FCC">Individual nations enforce this prohibition in their communication laws. In the United States, Federal Communications Commission (FCC) regulations make it a felony to operate a spark transmitter: {{cite web
| title= Section 2.201: Emission, modulation, and transmission characteristics, footnote (f)
| work= Code of Federal Regulations, Title 47, Chapter I, Subchapter A, Part 2, Subpart C
| publisher= US Government Publishing Office website
| date= 1 October 2007
| url= https://www.gpo.gov/fdsys/pkg/CFR-2007-title47-vol1/xml/CFR-2007-title47-vol1-sec2-201.xml
| access-date= 16 March 2018}}</ref><ref name="Schroeder"/>

==Theory of operation==
[[Electromagnetic wave]]s are radiated by [[electric charge]]s when they are [[acceleration|accelerated]].<ref name="Serway">{{cite book |last1= Serway |first1= Raymond |last2= Faughn |first2= Jerry |last3= Vuille |first3= Chris |title= College Physics |edition= 8th |publisher= Cengage Learning |date= 2008 |pages= 714 |url= https://books.google.com/books?id=CX0u0mIOZ44C&q=%22electromagnetic+wave%22+charge+acceleration&pg=PA714 |isbn= 978-0495386933}}</ref><ref name="Ellingson">{{cite book
| last1= Ellingson
| first1= Steven W.
| title= Radio Systems Engineering
| publisher= Cambridge University Press
| date= 2016
| pages= 16–17
| url= https://books.google.com/books?id=QMKSDQAAQBAJ&q=%22radio+wave%22+time+varying+electric+current&pg=PA16
| isbn= 978-1316785164
}}</ref> [[Radio wave]]s, electromagnetic waves of radio [[frequency]], can be generated by time-varying [[electric current]]s, consisting of [[electron]]s flowing through a conductor which suddenly change their velocity, thus accelerating.<ref name="Ellingson"/><ref name="Nahin1">{{cite book
| last1= Nahin
| first1= Paul J.
| title= The Science of Radio: with MATLAB and Electronics Workbench demonstrations
| edition= 2nd
| publisher= Springer Science and Business Media
| date= 2001
| pages= 27–28
| url= https://books.google.com/books?id=V1GBW6UD4CcC&q=%22electromagnetic+waves%22+accelerated+&pg=PA27
| isbn= 978-0387951508
}}</ref> 

An electrically charged [[capacitance]] discharged through an [[electric spark]] across a [[spark gap]] between two conductors was the first device known which could generate radio waves.<ref name="Aitken2014" />{{rp|p.3}} The spark itself doesn't produce the radio waves, it merely serves as a fast acting [[switch]] to excite [[resonant]] [[radio frequency]] oscillating [[electric current]]s in the conductors of the attached circuit. The conductors radiate the energy in this oscillating current as radio waves. 

Due to the inherent [[inductance]] of circuit conductors, the discharge of a capacitor through a low enough resistance (such as a spark) is [[oscillatory]]; the charge flows rapidly back and forth through the spark gap for a brief period, charging the conductors on each side alternately positive and negative, until the oscillations die away.<ref name="CodellaSparkRadio">{{cite web
| last= Codella
| first= Christopher F.
| title= Spark Radio
| work= Ham Radio History
| publisher= C. F. Codella's private website
| date= 2016
| url= http://w2pa.net/HRH/spark-radio/
| access-date= 22 May 2018}}</ref><ref name="Fleming2">{{cite book
| last1= Fleming
| first1= John Archibald
| title= The Principles of Electric Wave Telegraphy
| publisher= Longmans Green and Co.
| date= 1906
| location= London
| pages= 15–16
| url= https://archive.org/stream/cu31924074412739#page/n45/mode/2up/
}}</ref>

[[File:Spark gap transmitter diagram (alt).png|thumb|upright=1.5|Pictorial diagram of a simple spark-gap transmitter from a 1917 boy's hobby book, showing examples of the early electronic components used. It is typical of the low-power transmitters homebuilt by thousands of amateurs during this period to explore the exciting new technology of radio.]]

A practical spark gap transmitter consists of these parts:<ref name="CodellaSparkRadio"/><ref name="Kennedy">{{cite web
| last= Kennedy
| first= Hal
| title= How spark transmitters work
| work= The history of QST Vol. 1 - Technology
| publisher= American Radio Relay League
| date= 1990
| url= http://www.arrl.org/files/file/History/History%20of%20QST%20Volume%201%20-%20Technology/Kennedy%20N4GG.pdf
| access-date= 27 March 2018}}</ref><ref name="Morecroft">{{cite book
| last1= Morecroft
| first1= John H.
| title= Principles of Radio Communication
| publisher= John Wiley and Sons
| date= 1921
| location= New York
| pages= [https://archive.org/details/principlesofradi00moreuoft/page/275 275]–279
| url= https://archive.org/details/principlesofradi00moreuoft
}}</ref><ref name="Hyder">{{cite journal
  | last1  = Hyder
  | first1 = Harry R.
  | title = The final days of ham spark
  | journal = QST
  | volume = 
  | issue = 
  | pages = 29–32
  | publisher = American Radio Relay League
  | date = March 1992
  | url = http://rfcec.com/RFCEC/Section-3%20-%20Fundamentals%20of%20RF%20Communication-Electronics/13%20-%20HISTORY/Radio%20History%2002%20-%20The%20Final%20Days%20of%20Ham%20Spark%20(By%20Harry%20Hydir%20W7IV).pdf
  | issn = 
  | doi = 
  | id = 
  | accessdate = 5 February 2022}}</ref>
*A high-voltage [[transformer]], to transform the low-[[voltage]] electricity from the power source, a battery or electric outlet, to a high enough voltage (from a few [[kilovolt]]s to 75-100 kilovolts in powerful transmitters) to jump across the spark gap. The transformer charges the capacitor. In low-power transmitters powered by batteries this was usually an [[induction coil]] (Ruhmkorff coil).
*One or more [[resonant circuit]]s (tuned circuits or tank circuits) which create [[radio frequency]] electrical [[oscillation]]s when excited by the spark. A resonant circuit consists of a [[capacitor]] (in early days a type called a [[Leyden jar]]) which stores high-voltage electricity from the transformer, and a coil of wire called an [[inductor]] or tuning coil, connected together. The values of the capacitance and inductance determine the [[frequency]] of the radio waves produced.
**The earliest spark-gap transmitters before 1897 did not have a resonant circuit; the antenna performed this function, acting as a [[resonator]]. However, this meant that the electromagnetic energy produced by the transmitter was dissipated across a wide band, thereby limiting its effective range to a few kilometers at most.
**Most spark transmitters had two resonant circuits coupled together with an air core transformer called a ''[[resonant transformer]]'' or ''oscillation transformer''.<ref name="CodellaSparkRadio"/> This was called an ''inductively-coupled'' transmitter. The spark gap and capacitor connected to the [[primary winding]] of the transformer made one resonant circuit, which generated the oscillating current. The oscillating current in the primary winding created an oscillating [[magnetic field]] that induced current in the [[secondary winding]]. The antenna and ground were connected to the secondary winding. The capacitance of the antenna resonated with the secondary winding to make a second resonant circuit. The two resonant circuits were tuned to the same [[resonant frequency]]. The advantage of this circuit was that the oscillating current persisted in the antenna circuit even after the spark stopped, creating long, ringing, lightly damped waves, in which the energy was concentrated in a narrower [[bandwidth (signal processing)|bandwidth]], creating less interference to other transmitters.
*A [[spark gap]] which acts as a voltage-controlled [[switch]] in the resonant circuit, discharging the capacitor through the coil.
*An [[antenna (radio)|antenna]], a metal conductor such as an elevated wire, that radiates the power in the oscillating electric currents from the resonant circuit into space as [[radio wave]]s.
*A [[telegraph key]] to switch the transmitter on and off to communicate messages by [[Morse code]]

===Operation cycle===
The transmitter works in a rapid repeating cycle in which the capacitor is charged to a high voltage by the transformer and discharged through the coil by a spark across the spark gap.<ref name="CodellaSparkRadio"/><ref name="Nahin4">Nahin, Paul J. (2001) ''[https://books.google.com/books?id=V1GBW6UD4CcC&pg=PA45&dq=spark The Science of Radio: with MATLAB and Electronics Workbench demonstrations, 2nd Ed.]'', p. 38-43</ref> The impulsive spark excites the resonant circuit to "ring" like a bell, producing a brief oscillating current which is radiated as electromagnetic waves by the antenna.<ref name="CodellaSparkRadio"/> The transmitter repeats this cycle at a rapid rate, so the spark appeared continuous, and the radio signal sounded like a whine or buzz in a [[radio receiver]].

[[File:Massie spark gap.jpg|thumb|right|Demonstration of the restored 1907 [[Massie Wireless Station]] spark gap transmitter 
{{Listen
| filename    = Massie Wireless Station "PJ" Spark Sound.ogg
| title       = Audio of Massie spark gap transmission
| description = [[Morse code]] of "[[CQ (call)|CQ DE]] PJ"
| pos         = left
| plain       = yes}}
]]
#The cycle begins when current from the transformer charges up the capacitor, storing positive electric charge on one of its plates and negative charge on the other.  While the capacitor is charging the spark gap is in its nonconductive state, preventing the charge from escaping through the coil.
#When the voltage on the capacitor reaches the [[breakdown voltage]] of the spark gap, the air in the gap [[ionization|ionizes]], starting an [[electric spark]], reducing its [[Electrical resistance|resistance]] to a very low level (usually less than one [[ohm]]). This closes the circuit between the capacitor and the coil.
#The charge on the capacitor discharges as a current through the coil and spark gap. Due to the [[inductance]] of the coil when the capacitor voltage reaches zero the current doesn't stop but keeps flowing, charging the capacitor plates with an opposite polarity, until the charge is stored in the capacitor again, on the opposite plates. Then the process repeats, with the charge flowing in the opposite direction through the coil. This continues, resulting in oscillating currents flowing rapidly back and forth between the plates of the capacitor through the coil and spark gap.
#The resonant circuit is connected to the antenna, so these oscillating currents also flow in the antenna, charging and discharging it. The current creates an oscillating [[magnetic field]] around the antenna, while the voltage creates an oscillating [[electric field]]. These oscillating fields radiate away from the antenna into space as an [[electromagnetic wave]]; a radio wave.
#The energy in the resonant circuit is limited to the amount of energy originally stored in the capacitor. The radiated radio waves, along with the heat generated by the spark, uses up this energy, causing the oscillations to decrease quickly in [[amplitude]] to zero. When the oscillating electric current in the primary circuit has decreased to a point where it is insufficient to keep the air in the spark gap ionized, the spark stops, opening the resonant circuit, and stopping the oscillations. In a transmitter with two resonant circuits, the oscillations in the secondary circuit and antenna may continue some time after the spark has terminated. Then the transformer begins charging the capacitor again, and the whole cycle repeats.

The cycle is very rapid, taking less than a millisecond. With each spark, this cycle produces a radio signal consisting of an oscillating [[sinusoidal]] wave that increases rapidly to a high [[amplitude]] and decreases [[exponential decay|exponentially]] to zero, called a [[Damped wave (radio transmission)|damped wave]].<ref name="CodellaSparkRadio"/> The [[frequency]] <math>f</math> of the oscillations, which is the frequency of the emitted radio waves, is equal to the [[resonant frequency]] of the resonant circuit, determined by the [[capacitance]] <math>C</math> of the capacitor and the [[inductance]] <math>L</math> of the coil:
:<math>f = \frac {1}{2 \pi} \sqrt { \frac {1}{LC}} \,</math>
The transmitter repeats this cycle rapidly, so the output is a repeating string of damped waves. This is equivalent to a radio signal [[amplitude modulation|amplitude modulated]] with a steady frequency, so it could be [[demodulation|demodulated]] in a radio receiver by a [[Rectifier|rectifying]] [[amplitude modulation|AM]] [[detector (radio)|detector]], such as the [[crystal detector]] or [[Fleming valve]] used during the wireless telegraphy era. The [[frequency]] of repetition (spark rate) is in the [[audio frequency|audio]] range, typically 50 to 1000 sparks per second, so in a receiver's [[earphone]]s the signal sounds like a steady tone, whine, or buzz.<ref name="Kennedy"/>

In order to transmit information with this signal, the operator turns the transmitter on and off rapidly by tapping on a [[switch]] called a [[telegraph key]] in the primary circuit of the transformer, producing sequences of short (dot) and long (dash) strings of damped waves, to spell out messages in [[Morse code]]. As long as the key is pressed the spark gap fires repetitively, creating a string of pulses of radio waves, so in a receiver the keypress sounds like a buzz; the entire Morse code message sounds like a sequence of buzzes separated by pauses. In low-power transmitters the key directly breaks the primary circuit of the supply transformer, while in high-power transmitters the key operates a heavy duty [[relay]] that breaks the primary circuit.

===Charging circuit and spark rate===
The circuit which charges the capacitors, along with the spark gap itself, determines the ''spark rate'' of the transmitter, the number of sparks and resulting damped wave pulses it produces per second, which determines the tone of the signal heard in the receiver. The spark rate should not be confused with the ''frequency'' of the transmitter, which is the number of sinusoidal oscillations per second in each damped wave. Since the transmitter produces one pulse of radio waves per spark, the output power of the transmitter was proportional to the spark rate, so higher rates were favored. Spark transmitters generally used one of three types of power circuits:<ref name="CodellaSparkRadio"/><ref name="Kennedy"/><ref name="Sarkar">{{cite book
| last1= Sarkar
| first1= T. K.
| last2= Mailloux
| first2= Robert
| last3= Oliner
| first3= Arthur A.
| title= History of Wireless
| publisher= John Wiley and Sons
| date= 2006
| url= https://archive.org/stream/HistoryOfWireless#page/n260/mode/2up
| isbn= 978-0471783015
}}</ref>{{rp|p.359–362}}

====Induction coil====
An [[induction coil]] (Ruhmkorff coil) was used in low-power transmitters, usually less than 500 watts, often battery-powered.  An induction coil is a type of transformer powered by DC, in which a vibrating arm switch contact on the coil called an [[interrupter]] repeatedly breaks the circuit that provides current to the primary winding, causing the coil to generate pulses of high voltage. When the primary current to the coil is turned on, the primary winding creates a magnetic field in the iron core which pulls the springy interrupter arm away from its contact, opening the switch and cutting off the primary current. Then the magnetic field collapses, creating a pulse of high voltage in the secondary winding, and the interrupter arm springs back to close the contact again, and the cycle repeats. Each pulse of high voltage charged up the capacitor until the spark gap fired, resulting in one spark per pulse. Interrupters were limited to low spark rates of 20–100&nbsp;Hz, sounding like a low buzz in the receiver. In powerful induction coil transmitters, instead of a vibrating interrupter, a [[Induction coil#Mercury and electrolytic interrupters|mercury turbine interrupter]] was used. This could break the current at rates up to several thousand hertz, and the rate could be adjusted to produce the best tone.

====AC transformer====
In higher power transmitters powered by AC, a [[transformer]] steps the input voltage up to the high voltage needed. The sinusoidal voltage from the transformer is applied directly to the capacitor, so the voltage on the capacitor varies from a high positive voltage, to zero, to a high negative voltage. The spark gap is adjusted so sparks only occur near the maximum voltage, at peaks of the AC [[sine wave]], when the capacitor was fully charged. Since the AC sine wave has two peaks per cycle, ideally two sparks occurred during each cycle, so the spark rate was equal to twice the frequency of the AC power<ref name="Hyder" /> (often multiple sparks occurred during the peak of each half cycle). The spark rate of transmitters powered by 50 or 60&nbsp;Hz mains power was thus 100 or 120&nbsp;Hz. However higher audio frequencies cut through interference better, so in many transmitters the transformer was powered by a [[motor–generator|motor–alternator]] set, an [[electric motor]] with its shaft turning an [[alternator]], that produced AC at a higher frequency, usually 500&nbsp;Hz, resulting in a spark rate of 1000&nbsp;Hz.<ref name="Hyder" />

====Quenched spark gap====
The speed at which signals may be transmitted is naturally limited by the time taken for the spark to be extinguished. If, as described above, the conductive plasma does not, during the zero points of the alternating current, cool enough to extinguish the spark, a 'persistent spark' is maintained until the stored energy is dissipated, permitting practical operation only up to around 60 signals per second.{{citation needed|date=September 2024}} If active measures are taken to break the arc (either by blowing air through the spark or by lengthening the spark gap), a much shorter "quenched spark" may be obtained.{{citation needed|date=September 2024}} A simple quenched spark system still permits several oscillations of the capacitor circuit in the time taken for the spark to be quenched. With the spark circuit broken, the transmission frequency is solely determined by the antenna resonant circuit, which permits simpler tuning.

====Rotary spark gap====
In a transmitter with a "rotary" spark gap ''(below)'', the capacitor was charged by AC from a high-voltage transformer as above, and discharged by a spark gap consisting of electrodes spaced around a wheel which was spun by an electric motor, which produced sparks as they passed by a stationary electrode.<ref name="CodellaSparkRadio"/><ref name="Hyder" /> The spark rate was equal to the rotations per second times the number of spark electrodes on the wheel. It could produce spark rates up to several thousand hertz, and the rate could be adjusted by changing the speed of the motor. The rotation of the wheel was usually synchronized to the AC [[sine wave]] so the moving electrode passed by the stationary one at the peak of the sine wave, initiating the spark when the capacitor was fully charged, which produced a musical tone in the receiver. When tuned correctly in this manner, the need for external cooling or quenching airflow was eliminated, as was the loss of power directly from the charging circuit (parallel to the capacitor) through the spark.

==History==
{{Further|Timeline of radio|Invention of radio}}
The invention of the radio transmitter resulted from the convergence of two lines of research.

One was efforts by inventors to devise a system to transmit [[telegraph]] signals without wires. Experiments by a number of inventors had shown that electrical disturbances could be transmitted short distances through the air. However most of these systems worked not by radio waves but by [[electrostatic induction]] or [[electromagnetic induction]], which had too short a range to be practical.<ref name="Nahin2"/> In 1866 [[Mahlon Loomis]] claimed to have transmitted an electrical signal through the atmosphere between two 600 foot wires held aloft by kites on mountaintops 14 miles apart.<ref name="Nahin2">{{cite book
| last1= Nahin
| first1= Paul J.
| title= The Science of Radio: with MATLAB and Electronics Workbench demonstrations
| edition= 2nd
| publisher= Springer Science and Business Media
| date= 2001
| pages= 7
| url= https://books.google.com/books?id=V1GBW6UD4CcC&q=Mahlon+Loomis+Nathan+Stubblefield+inductive&pg=PA7
| isbn= 978-0387951508
}}</ref> [[Thomas Edison]] had come close to discovering radio in 1875; he had generated and detected radio waves which he called "etheric currents" experimenting with high-voltage spark circuits, but due to lack of time did not pursue the matter.<ref name="Sarkar" />{{rp|p.259–261}} [[David Edward Hughes]] in 1879 had also stumbled on radio wave transmission which he received with his [[carbon microphone]] detector, however he was persuaded that what he observed was [[electromagnetic induction|induction]].<ref name="Sarkar"/>{{rp|p.259–261}} Neither of these individuals are usually credited with the discovery of radio, because they did not understand the significance of their observations and did not publish their work before Hertz.

The other was research by physicists to confirm the theory of [[electromagnetism]] proposed in 1864 by Scottish physicist [[James Clerk Maxwell]], now called [[Maxwell's equations]]. Maxwell's theory predicted that a combination of oscillating [[electric field|electric]] and [[magnetic field]]s could travel through space as an "[[electromagnetic wave]]". Maxwell proposed that light consisted of electromagnetic waves of short wavelength, but no one knew how to confirm this, or generate or detect electromagnetic waves of other wavelengths. By 1883 it was theorized that accelerated electric charges could produce electromagnetic waves, and [[George Francis FitzGerald|George Fitzgerald]] had calculated the output power of a [[loop antenna]].<ref name="Fitzgerald1">Fitzgerald, George "On the energy lost by radiation from alternating electric currents", ''Report of the British Association for the Advancement of Science'', 1883, reprinted in {{cite book
| last1= Fitzgerald
| first1= George
| title= The Scientific Writings of the Late George Francis Fitzgerald
| publisher= Hodges, Figgis, and Co.
| date= 1902
| location= London
| pages= 128–129
| url= https://books.google.com/books?id=G0bPAAAAMAAJ&pg=PA128
}}</ref> Fitzgerald in a brief note published in 1883 suggested that electromagnetic waves could be generated practically by discharging a capacitor rapidly; the method used in spark transmitters,<ref name="Nahin3">{{cite book
| last1= Nahin
| first1= Paul J.
| title= The Science of Radio: with MATLAB and Electronics Workbench demonstrations
| edition= 2nd
| publisher= Springer Science and Business Media
| date= 2001
| pages= 18
| url= https://books.google.com/books?id=V1GBW6UD4CcC&q=%22&pg=PA18
| isbn= 978-0387951508
}}</ref><ref name="Fitzgerald2">Fitzgerald, George "On a method of producing electromagnetic disturbances of comparatively short wavelength", ''Report of the British Association for the Advancement of Science'', 1883, p.405, reprinted in {{cite book
| last1= Fitzgerald
| first1= George
| title= The Scientific Writings of the Late George Francis Fitzgerald
| publisher= Hodges, Figgis, and Co.
| date= 1902
| location= London
| pages= 129
| url= https://books.google.com/books?id=G0bPAAAAMAAJ&pg=PA129
}}. The text in full: "This is by utilizing the alternating currents produced when an accumulator is discharged through a small resistance. It would be possible to produce waves of ten meters wavelength, or even less"</ref> however there is no indication that this inspired other inventors.

The division of the history of spark transmitters into the different types below follows the organization of the subject used in many wireless textbooks.<ref name="Zenneck">{{cite book
| last= Zenneck
| first= Jonathan Adolf Wilhelm
| title= Wireless Telegraphy
| publisher= McGraw Hill Book Co.
| date= 1915
| location= New York
| pages= [https://archive.org/details/in.ernet.dli.2015.212404/page/n193 173]
| url= https://archive.org/details/in.ernet.dli.2015.212404
}} Translated from German by A. E. Selig. Zenneck describes the Marconi, Braun, and Wien transmitters on p. 173, and the early "lineal" or Hertz oscillators on p. 41.</ref>

===Hertzian oscillators===
German physicist [[Heinrich Hertz]] in 1887 built the first experimental spark gap transmitters during his historic experiments to demonstrate the existence of [[electromagnetic wave]]s predicted by [[James Clerk Maxwell]] in 1864, in which he discovered [[radio wave]]s,<ref name="Hertz1">Hertz, H., "On very rapid electric oscillations", ''Wiedemann's Annalen'', Vol. 31, p. 421, 1887 reprinted in {{cite book
| last1= Hertz
| first1= Heinrich
| title= Electric Waves: Being Researches on the Propagation of Electric Action with Finite Velocity Through Space
| publisher= Dover Publications
| date= 1893
| pages= [https://archive.org/details/electricwavesbe00hertgoog/page/n48 29]–53
| url= https://archive.org/details/electricwavesbe00hertgoog
| quote= heinrich hertz .
}} translated to English by D. E. Jones</ref>
<ref name="Hong">{{cite book
| last1= Hong
| first1= Sungook
| title= Wireless: From Marconi's Black-box to the Audion
| publisher= MIT Press
| date= 2010
| url= https://books.google.com/books?id=uJ0jEAAAQBAJ
| isbn= 9780262514194
}}</ref>{{rp|p.3-4}}<ref name="Baird">{{cite book
| last1= Baird
| first1= D.
| last2= Hughes
| first2= R.I.
| last3= Nordmann
| first3= A.
| title= Heinrich Hertz: Classical Physicist, Modern Philosopher
| publisher= Springer Science and Business Media
| date= 2013
| pages= 51–53
| url= https://books.google.com/books?id=9iEyBwAAQBAJ&q=oscillator+transmitter&pg=PA52
| isbn= 978-9401588553
}}</ref><ref name="Sarkar" />{{rp|p.19, 260, 331-332}} which were called "Hertzian waves" until about 1910. Hertz was inspired to try spark excited circuits by experiments with "Reiss spirals", a pair of flat spiral [[inductor]]s with their conductors ending in spark gaps. A [[Leyden jar]] capacitor discharged through one spiral, would cause sparks in the gap of the other spiral.

<gallery mode="packed" heights="150">
File:Heinrich Hertz discovering radio waves.png|Heinrich Hertz discovering radio waves with his spark oscillator ''(at rear)''
File:Hertz drawing of his spark transmitter 1888.png|Hertz's drawing of one of his spark oscillators. ''(A,A')'' antenna, ''(J)'' induction coil
File:Hertzian spark radio transmitter 1902.jpg|Hertzian spark oscillator, 1902. Visible are antenna consisting of 2 wires ending in metal plates ''(E)'', spark gap ''(D)'', induction coil ''(A)'', auto battery ''(B)'', and [[telegraph key]] ''(C)''.
File:Hertz spark gap transmitter and parabolic antenna.png|Hertz's 450&nbsp;MHz transmitter; a 26&nbsp;cm dipole with spark gap at focus of a sheet metal parabolic reflector
File:Microwave Apparatus - Jagadish Chandra Bose Museum - Bose Institute - Kolkata 2011-07-26 4051.JPG|[[Jagadish Chandra Bose]] in 1894 was the first person to produce [[millimeter wave]]s; his spark oscillator ''(in box, right)'' generated 60&nbsp;GHz (5&nbsp;mm) waves using 3&nbsp;mm metal ball resonators.
File:Lodge spark oscillator ball 1894.jpg|Spark oscillator ball used by [[Oliver Lodge]] in 1894 to generate 25&nbsp;cm ( 1.2&nbsp;GHz) microwaves
</gallery>
[[File:Hertz first oscillator.png|thumb|upright=3|center|Hertz's first oscillator: a pair of one meter copper wires with a 7.5 mm spark gap between them, ending in 30 cm zinc spheres. When 20,000 volt pulses from an induction coil ''(not shown)'' was applied, it produced waves at a [[frequency]] of roughly 50 MHz.]]

See circuit diagram. Hertz's transmitters consisted of a [[dipole antenna]] made of a pair of collinear metal rods of various lengths with a [[spark gap]] ''(S)'' between their inner ends and metal balls or plates for [[capacitance]] ''(C)'' attached to the outer ends.<ref name="Hertz1"/><ref name="Sarkar" />{{rp|p.19, 260, 331-332}}<ref name="Baird"/> The two sides of the antenna were connected to an [[induction coil]] (Ruhmkorff coil) ''(T)'' a common lab power source which produced pulses of high voltage, 5 to 30&nbsp;kV. In addition to radiating the waves, the antenna also acted as a [[harmonic oscillator]] ([[resonator]]) which generated the oscillating currents. High-voltage pulses from the induction coil ''(T)'' were applied between the two sides of the antenna. Each pulse stored electric charge in the capacitance of the antenna, which was immediately discharged by a spark across the spark gap. The spark excited brief oscillating [[standing wave]]s of current between the sides of the antenna. The antenna radiated the energy as a momentary pulse of radio waves; a [[Damped wave (radio transmission)|damped wave]]. The frequency of the waves was equal to the [[resonant frequency]] of the antenna, which was determined by its length; it acted as a [[half-wave dipole]], which radiated waves roughly twice the length of the antenna (for example a dipole 1 meter long would generate 150 MHz radio waves). Hertz detected the waves by observing tiny sparks in micrometer spark gaps ''(M)'' in loops of wire which functioned as resonant receiving antennas. [[Oliver Lodge]] was also experimenting with spark oscillators at this time and came close to discovering radio waves before Hertz, but his focus was on waves on wires, not in free space.<ref name="Lee1">{{cite book
| last= Lee
| first= Thomas H.
| title= The Design of CMOS Radio-Frequency Integrated Circuits
| edition= 2nd
| publisher= Cambridge University Press
| date= 2004
| location= UK
| pages= 34–36
| url= https://books.google.com/books?id=io1hL48OqBsC&q=lodge+syntony&pg=PA34
| isbn= 978-0521835398}}</ref><ref name="Sarkar" />{{rp|p.226}}

[[File:Hertz transmitter and receiver - English.svg|thumb|upright=1.3|Circuit of Hertz's spark oscillator and receiver. The interrupter ''(I)'' and capacitor in the primary circuit of the induction coil produced a continuous string of damped waves.  Hertz often just used a pushbutton switch, which created a single spark and pulse of radio waves when pushed, resulting in a single spark in his receiver.]]

Hertz and the first generation of physicists who built these "Hertzian oscillators", such as [[Jagadish Chandra Bose]], [[Lord Rayleigh]], [[George Francis FitzGerald|George Fitzgerald]], [[Frederick Trouton]], [[Augusto Righi]] and [[Oliver Lodge]], were mainly interested in radio waves as a [[scientific phenomenon]], and largely failed to foresee its possibilities as a communication technology.<ref name="McNicol">{{cite book
| last1= Donald
| first1= McNicol
| title= Radio's Conquest of Space: The experimental rise of radio communication
| publisher= Murray Hill Books, Inc.
| date= 1946
| isbn= 9780405060526
| url= https://archive.org/details/radiosconquestof00mcnirich/page/53 
}}</ref>{{rp|p.54, 98}}<ref name="Hong" />{{rp|p.5-9,22}}<ref name="Sarkar" />{{rp|p.260, 263-265}}<ref name="Coe1">{{cite book
| last1= Coe
| first1= Lewis
| title= Wireless Radio: A History
| publisher= McFarland
| date= 2006
| pages= 4–6, 13
| url= https://books.google.com/books?id=W1JAeg1PiWIC&pg=PA4
| isbn= 978-0786426621
}}</ref>  Due to the influence of Maxwell's theory, their thinking was dominated by the similarity between radio waves and light waves; they thought of radio waves as an invisible form of light.<ref name="Hong" />{{rp|p.5-9,22}}<ref name="Sarkar" />{{rp|p.260, 263-265}} By analogy with light, they assumed that radio waves only traveled in straight lines, so they thought radio transmission was limited by the visual [[horizon]] like existing optical signalling methods such as [[semaphore]], and therefore was not capable of longer distance communication.<ref name="Lee1"/><ref name="Weightman">{{cite book
| last1= Weightman
| first1= Gavin
| title= Signor Marconi's Magic Box: The Most Remarkable Invention Of The 19th Century & The Amateur Inventor Whose Genius Sparked A Revolution
| publisher= Da Capo Press
| date= 2009
| pages= 52
| url= https://books.google.com/books?id=UCdvcmMTpGgC&pg=PT52
| isbn= 978-0786748549
}}</ref><ref name="Gregersen">{{cite book
| last1= Gregersen
| first1= Erik
| title= The Britannica Guide to Sound and Light
| publisher= The Rosen Publishing Group
| date= 2011
| pages= 159
| url= https://books.google.com/books?id=EMBelfRf6dkC&q=Marconi+%22ground+wave%22+%22Kennelly+heaviside+layer%22+%22straight+lines%22&pg=PA169
| isbn= 978-1615303007
}}</ref> As late as 1894 Oliver Lodge speculated that the maximum distance Hertzian waves could be transmitted was a half mile.<ref name="Hong" />{{rp|p.5-9,22}}

To investigate the similarity between radio waves and [[light wave]]s, these researchers concentrated on producing short [[wavelength]] high-frequency waves with which they could duplicate classic [[optics]] experiments with radio waves, using [[quasioptics|quasioptical]] components such as [[Prism (optics)|prism]]s and [[Lens (optics)|lens]]es made of [[paraffin wax]], [[sulfur]], and [[pitch (resin)|pitch]] and wire [[diffraction grating]]s.<ref name="Sarkar" />{{rp|p.476-484}} Their short antennas generated radio waves in the [[very high frequency|VHF]], [[ultra high frequency|UHF]], or [[microwave]] bands. In his various experiments, Hertz produced waves with frequencies from 50 to 450&nbsp;MHz, roughly the frequencies used today by broadcast [[television transmitter]]s. Hertz used them to perform historic experiments demonstrating [[standing wave]]s, [[refraction]], [[diffraction]], [[polarization (waves)|polarization]] and [[interference (wave propagation)|interference]] of radio waves.<ref name="Hertz2">Hertz, H., "On radiation", ''Wiedemann's Annalen'', Vol. 36, December 13, 1988, p. 769, reprinted in {{cite book
| last1= Hertz
| first1= Heinrich
| title= Electric Waves: Being Researches on the Propagation of Electric Action with Finite Velocity Through Space
| publisher= Dover Publications
| date= 1893
| pages= [https://archive.org/details/electricwavesbe00hertgoog/page/n191 172]–185
| url= https://archive.org/details/electricwavesbe00hertgoog
}} translated to English by D. E. Jones</ref><ref name="Sarkar" />{{rp|p.19, 260, 331-332}} He also measured the speed of radio waves, showing they traveled at the same speed as light. These experiments established that light and radio waves were both forms of Maxwell's [[electromagnetic wave]]s, differing only in frequency. [[Augusto Righi]] and [[Jagadish Chandra Bose]] around 1894 generated [[microwave]]s of 12 and 60&nbsp;GHz respectively, using small metal balls as resonator-antennas.<ref name="Bose">{{cite journal
| last1= Bose
| first1= Jagadish Chandra
| title= On a complete apparatus for the study of the properties of electric waves
| journal= The London, Edinburgh, and Dublin Philosophical Magazine
| volume= 43
| issue= 5
| pages= 55–88
| date= January 1897
| url= https://books.google.com/books?id=OLYvAQAAIAAJ&pg=PA55
| doi =10.1080/14786449708620959
| access-date= January 30, 2018}}</ref><ref name="Sarkar" />{{rp|p.291-308}}

The high frequencies produced by Hertzian oscillators could not travel beyond the horizon. The dipole resonators also had low capacitance and couldn't store much [[electric charge|charge]], limiting their power output.<ref name="Hong" />{{rp|p.5-9,22}} Therefore, these devices were not capable of long distance transmission; their reception range with the primitive receivers employed was typically limited to roughly 100 yards (100 meters).<ref name="Hong" />{{rp|p.5-9,22}}
{{Clear}}

===Non-syntonic transmitters===
{{Quote
|text=I could scarcely conceive it possible that [radio's] application to useful purposes could have escaped the notice of such eminent scientists.
|author=Guglielmo Marconi<ref name="Beauchamp1"/>
}}
Italian radio pioneer [[Guglielmo Marconi]] was one of the first people to believe that radio waves could be used for long distance communication, and singlehandedly developed the first practical [[radiotelegraphy]] [[transmitter]]s and [[radio receiver|receivers]],<ref name="Coe1"/><ref name="Lee2">{{cite book
| last= Lee
| first= Thomas H.
| title= The Design of CMOS Radio-Frequency Integrated Circuits
| edition= 2nd
| publisher= Cambridge University Press
| date= 2004
| location= UK
| pages= 37–39
| url= https://books.google.com/books?id=io1hL48OqBsC&q=Marconi+Tesla&pg=PA37
| isbn= 978-0521835398}}</ref><ref name="Hong" />{{rp|ch.1&2}} mainly by combining and tinkering with the inventions of others. Starting at age 21 on his family's estate in Italy, between 1894 and 1901 he conducted a long series of experiments to increase the transmission range of Hertz's spark oscillators and receivers.<ref name="Beauchamp1">{{cite book
| last1= Beauchamp
| first1= Ken
| title= History of Telegraphy
| publisher= IET
| date= 2001
| pages= 186–190
| url= https://books.google.com/books?id=k3XCkncd83AC&pg=PA186
| isbn= 978-0852967928
}}</ref>

{{multiple image
| align= center
| direction= horizontal
| header= Evolution of Marconi's monopole antenna from Hertz's dipole antenna
| image1= Hertz spark radio transmitter 1887.png
| caption1= Hertz's dipole oscillator
| width1= 200
| image2= Marconi capacity area transmitter 1896.png
| caption2= Marconi first tried enlarging the dipole antenna with 6×6&nbsp;foot metal sheet "capacity areas" ''(r)'', 1895<ref name="patentGB12039"/> Metal sheets and spark balls not shown to scale.
| width2= 240
| image3= Marconi first monopole transmitter 1896.png
| caption3= Marconi's first monopole antenna transmitter, 1895. One side of spark gap grounded, the other attached to a metal plate ''(W)''.<ref name="patentGB12039"/>
| width3= 140
| image4= Marconi's first radio transmitter.jpg
| caption4= Re-creation of Marconi's first monopole transmitter
| width4= 139
| image5= Early vertical wire antennas.svg
| caption5= Early vertical antennas. ''(A)'' Marconi found suspending the metal plate "capacity area" high above the ground increased range. ''(B)'' He found that a simple elevated wire worked just as well. ''(C-F)'' Later researchers found that multiple parallel wires were a better way to increase capacitance. "Cage antennas" ''(E-F)'' distributed current more equally between wires, reducing resistance
| width5= 353
| footer =
}}
He was unable to communicate beyond a half-mile until 1895, when he discovered that the range of transmission could be increased greatly by replacing one side of the Hertzian dipole antenna in his transmitter and receiver with a connection to [[ground (electricity)|Earth]] and the other side with a long wire antenna suspended high above the ground.<ref name="Hong" />{{rp|p.20-21}}<ref name="Coe1"/><ref name="Aitken1985" />{{rp|195-218}}<ref name="Huurdeman1"/> These antennas functioned as [[quarter-wave antenna|quarter-wave]] [[monopole antenna]]s.<ref name="Visser1">{{cite book
| last1= Visser
| first1= Hubregt J.
| title= Array and Phased Array Antenna Basics
| publisher= John Wiley and Sons
| date= 2006
| pages= 30–33
| url= https://books.google.com/books?id=qv9luhVAKAQC&q=Marconi+monopole+%22quarter+wave%22&pg=PA31
| isbn= 978-0470871188
}}</ref> The length of the antenna determined the wavelength of the waves produced and thus their frequency. Longer, lower frequency waves have less attenuation with distance.<ref name="Visser1"/> As Marconi tried longer antennas, which radiated lower frequency waves, probably in the [[medium frequency|MF]] band around 2&nbsp;MHz,<ref name="Huurdeman1">{{cite book
| last1= Huurdeman
| first1= Anton A.
| title= The Worldwide History of Telecommunications
| publisher= John Wiley and Sons
| date= 2003
| pages= 207–209
| url= https://books.google.com/books?id=SnjGRDVIUL4C&q=Marconi&pg=PA208
| isbn= 978-0471205050
}}</ref> he found that he could transmit further.<ref name="Beauchamp1"/> Another advantage was that these vertical antennas radiated [[vertical polarization|vertically polarized]] waves, instead of the [[horizontal polarization|horizontally polarized]] waves produced by Hertz's horizontal antennas.<ref name="Dosi">{{cite book
| last1= Dosi
| first1= Giovanni
| last2= Teece
| first2= David J.
| last3= Chytry
| first3= Josef
| title= Understanding Industrial and Corporate Change
| publisher= OUP Oxford
| date= 2004
| pages= 251
| url= https://books.google.com/books?id=2qfRJuJ0QDgC&q=Marconi+%22vertically+polarized%22&pg=PA251
| isbn= 978-0191533457
}}</ref> These longer vertically polarized waves could travel beyond the horizon, because they propagated as a [[ground wave]] that followed the contour of the Earth. Under certain conditions they could also reach beyond the horizon by reflecting off layers of charged particles ([[ion]]s) in the upper atmosphere, later called [[skywave]] propagation.<ref name="Gregersen"/>  Marconi did not understand any of this at the time; he simply found empirically that the higher his vertical antenna was suspended, the further it would transmit.
{{multiple image
| align= center
| direction= horizontal
| header =
| image1= Guglielmo Marconi 1901 wireless signal.jpg
| caption1= Marconi in 1901 with his early spark transmitter ''(right)'' and coherer receiver ''(left)'', which recorded the Morse code symbols with an ink line on a paper tape.
| width1= 264
| image2= Post Office Engineers.jpg
| caption2= British Post Office officials examining Marconi's transmitter ''(center)'' and receiver ''(bottom)'' during a demonstration 1897. The pole supporting the vertical wire antenna is visible at center.
| width2= 244
| image3= Marconi 1897 spark gap transmitter.jpg
| caption3= Marconi's transmitter in July 1897. ''(left)'' 4 ball Righi spark gap, ''(right)'' Induction coil, telegraph key, and battery box.
| width3= 250
| image4= Spark gap transmitter.jpg
| caption4= French non-syntonic transmitter used for ship-to-shore communication around 1900. It had a range of about {{convert|10|km|miles|sigfig=2}}.
| width4= 142
| footer =
}}
After failing to interest the Italian government, in 1896 Marconi moved to England, where [[William Preece]] of the British [[General Post Office]] funded his experiments.<ref name="Visser1"/><ref name="Huurdeman1"/><ref name="Beauchamp1"/> Marconi applied for a patent on his radio system 2 June 1896,<ref name="patentGB12039">British patent 189612039 Marconi, Guglielmo ''[https://worldwide.espacenet.com/publicationDetails/biblio?DB=EPODOC&II=0&ND=3&adjacent=true&locale=en_EP&FT=D&date=18970702&CC=GB&NR=189612039A&KC=A# Improvements in transmitting electrical impulses and signals, and in apparatus therefore]'', Applied: 2 June 1896, full specification: 2 March 1897, accepted: 2 July 1897. British patents allowed the full specification to be submitted after the application. Marconi's monopole antenna did not appear in his initial June 1896 application but in his March 1897 specification. Corresponding US patent 586193, Marconi, Guglielmo, ''[https://patents.google.com/patent/US586193 Transmitting electrical signals]'', filed 7 December 1896, accepted: 13 July 1897</ref> often considered the first wireless patent.<ref name="Sarkar" />{{rp|p.352-353, 355-358}}<ref name="Morse2">[https://archive.org/stream/radiobeamandbroa029214mbp#page/n29/mode/2up/search/marconi Morse (1925) ''Radio: Beam and Broadcast'', p. 24-26]</ref> In May 1897 he transmitted 14&nbsp;km (8.7&nbsp;miles),<ref name="Visser1"/> on 27 March 1899 he transmitted across the [[English Channel]], 46&nbsp;km (28&nbsp;miles),<ref name="Beauchamp1"/> in fall 1899 he extended the range to 136&nbsp;km (85&nbsp;miles),<ref name="Hong" />{{rp|p.60-61}} and by January 1901 he had reached 315&nbsp;km (196&nbsp;miles). These demonstrations of wireless [[Morse code]] communication at increasingly long distances convinced the world that radio, or "wireless telegraphy" as it was called, was not just a scientific curiosity but a commercially useful communication technology.

In 1897 Marconi started a company to produce his radio systems, which became the [[Marconi Wireless Telegraph Co]]mpany.<ref name="Visser1"/><ref name="Beauchamp1"/> and radio communication began to be used commercially around 1900.  His first large contract in 1901 was with the insurance firm [[Lloyd's of London]] to equip their ships with wireless stations. Marconi's company dominated [[marine radio]] throughout the spark era. Inspired by Marconi, in the late 1890s other researchers also began developing competing spark radio communication systems; [[Alexander Stepanovich Popov|Alexander Popov]] in Russia, [[Eugène Ducretet]] in France, [[Reginald Fessenden]] and [[Lee de Forest]] in America,<ref name="Hempstead"/> and [[Karl Ferdinand Braun]], [[Adolf Slaby]], and [[Georg von Arco]] in Germany who in 1903 formed the [[Telefunken]] Co., Marconi's chief rival.<ref name="Huurdeman2">[https://books.google.com/books?id=SnjGRDVIUL4C&pg=PA212&dq=Arco+Slaby+Braun+Popov+Ducretet Huurdeman, Anton (2003) ''The Worldwide History of Telecommunications'', p. 212-215]</ref><ref name="Burns1">{{cite book
| last1= Burns
| first1= Russell W.
| title= Communications: An International History of the Formative Years
| publisher= Institute of Electrical Engineers
| date= 2004
| pages= 313–329
| url= https://books.google.com/books?id=7eUUy8-VvwoC&q=Popov+Lodge+Muirhead+Slaby+Arco+Braun+%22De+Forest%22+Fessenden&pg=PA313
| isbn= 978-0863413278
}}</ref>

====Disadvantages====
[[File:Marconi spark gap transmitter 1896.svg|thumb|Circuit of Marconi's monopole transmitter and all other transmitters prior to 1897.]]

The primitive transmitters prior to 1897 had no [[resonant circuit]]s (also called LC circuits, tank circuits, or tuned circuits), the spark gap was in the antenna, which functioned as the resonator to determine the frequency of the radio waves.<ref name="Beauchamp1"/><ref name="Nahin5">Nahin, Paul J. (2001) ''[https://books.google.com/books?id=V1GBW6UD4CcC&pg=PA46&dq=spark The Science of Radio: with MATLAB and Electronics Workbench demonstrations, 2nd Ed.]'', p. 46</ref><ref name="Sarkar" />{{rp|p.352-353, 355-358}}<ref name="Thrower">{{cite conference
| last1= Thrower
| first1= K. R.
| title= History of tuning
| conference= Proceedings of the 1995 International Conference on 100 Years of Radio
| publisher= Institute of Engineering Technology
| date= 5 September 1995
| location= London
| url= https://ieeexplore.ieee.org/document/491801
| doi= 10.1049/cp:19950799
| isbn= 0-85296-649-0
| access-date= 20 June 2018}} [https://www.nonstopsystems.com/radio/pdf-hell/article-IC-1995-Thrower.pdf archived]</ref> These were called "unsyntonized" or "plain antenna" transmitters.<ref name="Sarkar" />{{rp|p.352-353, 355-358}}<ref name="Marriott1">{{cite journal
| last1= Marriott
| first1= Robert H.
| title= United States Radio Development
| journal= Proceedings of the I.R.E.
| volume= 5
| issue= 3
| pages= 179–188
| date= June 1917
| url= https://books.google.com/books?id=NNU1AQAAMAAJ&q=%22plain+antenna%22+%22coupled+circuit%22+transmitter&pg=PA189
| access-date= 8 March 2018}}</ref>

The average power output of these transmitters was low, because due to its low capacitance the antenna was a highly [[damping ratio|damped]] oscillator (in modern terminology, it had very low [[Q factor]]).<ref name="Aitken2014">{{cite book
| last1= Aitken
| first1= Hugh G.J.
| title= The Continuous Wave: Technology and American Radio, 1900-1932
| publisher= Princeton University Press
| date= 2014
| url= https://books.google.com/books?id=ebr_AwAAQBAJ&pg=PA3
| isbn= 978-1400854608
}}</ref>{{rp|p.4–7, 32–33}} During each spark the energy stored in the antenna was quickly radiated away as radio waves, so the oscillations decayed to zero quickly.<ref name="Ashley1">[https://books.google.com/books?id=pK-EAAAAIAAJ&pg=PA34 Ashley, Hayward (1912) ''Wireless Telegraphy and Wireless Telephony: An understandable presentation of the science of wireless transmission of intelligence''], p. 34-36</ref> The radio signal consisted of brief pulses of radio waves, repeating tens or at most a few hundreds of times per second, separated by comparatively long intervals of no output.<ref name="Sarkar" />{{rp|p.352-353, 355-358}} The power radiated was dependent on how much [[electric charge]] could be stored in the antenna before each spark, which was proportional to the [[capacitance]] of the antenna. To increase their capacitance to ground, antennas were made with multiple parallel wires, often with capacitive toploads, in the "harp", "cage", "[[umbrella antenna|umbrella]]", "inverted-L", and "[[T-antenna|T]]" antennas characteristic of the "spark" era.<ref name="CodellaAerials">{{cite web
| last= Codella
| first= Christopher F.
| title= Aerials, Attachments, and Audibility
| work= Ham Radio History
| publisher= Codella's private website
| date= 2016
| url= http://w2pa.net/HRH/aerials-attachments-and-audibility/
| access-date= 22 May 2018}}</ref> The only other way to increase the energy stored in the antenna was to charge it up to very high voltages.<ref name="Jansky1">{{cite book
| last1= Jansky
| first1= Cyril Methodius
| title= Principles of Radiotelegraphy
| publisher= McGraw-Hill Book Co.
| date= 1919
| location= New York
| pages= [https://archive.org/details/principlesradio01jansgoog/page/n179 165]–167
| url= https://archive.org/details/principlesradio01jansgoog
}}</ref><ref name="Sarkar" />{{rp|p.352-353, 355-358}} However the voltage that could be used was limited to about 100&nbsp;kV by [[corona discharge]] which caused charge to leak off the antenna, particularly in wet weather, and also energy lost as heat in the longer spark.

[[File:Spark gap transmitter bandwidth - A.png |thumb |right |Emission bandwidth of a spark gap transmitter showing signal strength versus wavelength in meters]]
A more significant drawback of the large [[damping ratio|damping]] was that the radio transmissions were electrically "noisy"; they had a very large [[bandwidth (signal processing)|bandwidth]].<ref name="CodellaSparkRadio"/><ref name="Hong" />{{rp|p.90-93}}<ref name="Beauchamp1"/><ref name="Aitken1985"/>{{rp|72-75}} These transmitters did not produce waves of a single [[frequency]], but a continuous band of frequencies.<ref name="Aitken1985">{{cite book |last1= Aitken |first1= Hugh G.J. |title= Syntony and Spark: The Origins of Radio |publisher= Princeton Univ. Press |date= 1985 |url= https://books.google.com/books?id=Mez_AwAAQBAJ&q=Fourier+noise&pg=PA72 |isbn= 978-1400857883}}</ref>{{rp|72-75}}<ref name="Hong" />{{rp|p.90-93}} They were essentially [[radio noise]] sources radiating energy over a large part of the [[radio spectrum]], which made it impossible for other transmitters to be heard.<ref name="Kennedy"/> When multiple transmitters attempted to operate in the same area, their broad signals overlapped in frequency and [[radio frequency interference|interfered]] with each other.<ref name="Beauchamp1"/><ref name="Thrower"/> The [[radio receiver]]s used also had no resonant circuits, so they had no way of selecting one signal from others besides the broad resonance of the antenna, and responded to the transmissions of all transmitters in the vicinity.<ref name="Thrower"/> An example of this interference problem was an embarrassing public debacle in August 1901 when Marconi, [[Lee de Forest]], and [[Greenleaf Whittier Pickard|G. W. Pickard]] attempted to report the New York Yacht Race to newspapers from ships with their untuned spark transmitters.<ref name="Lee3">[https://books.google.com/books?id=io1hL48OqBsC&pg=PA6&dq=%22yacht+race%22 Lee, Thomas H. 2004 ''The Design of CMOS Radio-Frequency Integrated Circuits, 2nd Ed.'', p. 6-7]</ref><ref name="Howeth1">{{cite book
| last1= Howeth
| first1= L. S.
| title= The History of Communications - Electronics in the U.S. Navy
| publisher= U.S. Navy
| date= 1963
| pages= [https://archive.org/details/historyofcommuni00howe/page/38 38]–39
| url= https://archive.org/details/historyofcommuni00howe
}}</ref><ref name="ElectricalWorld1901">{{cite journal
| title= Reporting the yacht races by wireless telegraph
| journal= Electrical World
| volume= 38
| issue= 15
| pages= 596–597
| date= October 12, 1901
| url= https://books.google.com/books?id=ntlQAAAAYAAJ&q=%22De+forest%22+marconi+interference&pg=PA596
| access-date= 8 March 2018}}</ref> The Morse code transmissions interfered, and the reporters on shore failed to receive any information from the garbled signals.
{{Clear}}

===Syntonic transmitters===
[[File:British patent 11575 Lodge IMPROVEMENTS IN SYNTONIZED TELEGRAPHY May 10 1897 fig 3 & 4.png|thumb|Transmitter ''(bottom)'' and receiver ''(top)'' of the first "syntonic" radio system, from Lodge's 1897 patent<ref name="Patent11575"/>]]

It became clear that for multiple transmitters to operate, some system of "selective signaling"<ref name="Ashley2">{{cite book
| last1= Ashley
| first1= Charles Grinnell
| last2= Hayward
| first2= Charles Brian
| title= Wireless telegraphy and wireless telephony: An understandable presentation of the science of wireless transmission of intelligence
| publisher= American School of Correspondence
| date= 1912
| pages= [https://archive.org/details/wirelesstelegra00haywgoog/page/n52 38]
| url= https://archive.org/details/wirelesstelegra00haywgoog
| quote= selective signaling.
}}</ref><ref name="Kennelly">{{cite book
| last1= Kennelly
| first1= Arthur Edwin
| title= Wireless Telegraphy: An Elementary Treatise
| publisher= Moffat, Yard, and Co.
| date= 1906
| location= New York
| pages= [https://archive.org/details/wirelesstelegra01kenngoog/page/n184 173]–180
| url= https://archive.org/details/wirelesstelegra01kenngoog
| quote= selective signaling.
}}</ref> had to be devised to allow a receiver to select which transmitter's signal to receive, and reject the others. In 1892 [[William Crookes]] had given an influential<ref name="Aitken">"Crookes’s article was read very widely—and more than that, attended to and remembered—both in Europe and in the United States; there is hardly one figure important in the early days of radio who does not at some point in his memoirs or correspondence refer to the article of 1892 as having made a difference."   [https://books.google.com/books?id=Mez_AwAAQBAJ&pg=PA115&dq=vertical+antenna Aitken, Hugh (2014) ''Syntony and Spark: The origins of radio'', p. 111-116]</ref> lecture<ref name="Crookes">{{cite journal
| last1= Crookes
| first1= William
| title= Some Possibilities of Electricity
| journal= The Fortnightly Review
| volume= 51
| pages= 174–176
| date= February 1, 1892
| url= http://www.tfo.upm.es/ImperialismoWeb/ArtCrookes.htm
| access-date= August 19, 2015
| archive-date= September 29, 2018
| archive-url= https://web.archive.org/web/20180929160315/http://www.tfo.upm.es/ImperialismoWeb/ArtCrookes.htm
| url-status= dead
}}</ref> on radio in which he suggested using [[resonance]] (then called ''syntony'') to reduce the bandwidth of transmitters and receivers.<ref name="Sarkar" />{{rp|p.352-353, 355-358}} Using a [[resonant circuit]] (also called tuned circuit or tank circuit) in transmitters would narrow the [[bandwidth (signal processing)|bandwidth]] of the radiated signal, it would occupy a smaller range of frequencies around its center frequency, so that the signals of transmitters "tuned" to transmit on different frequencies would no longer overlap. A receiver which had its own resonant circuit could receive a particular transmitter by "tuning" its [[resonant frequency]] to the frequency of the desired transmitter, analogously to the way one musical instrument could be tuned to resonance with another.<ref name="Ashley2"/> This is the system used in all modern radio.

During the period 1897 to 1900 wireless researchers realized the advantages of "syntonic" or "tuned" systems, and added [[capacitor]]s ([[Leyden jar]]s) and [[inductor]]s (coils of wire) to transmitters and receivers, to make resonant circuits (tuned circuits, or tank circuits).<ref name="Aitken1985"/>{{rp|p. 125-136, 254-255, 259}} [[Oliver Lodge]], who had been researching electrical resonance for years,<ref name="Aitken1985" />{{rp|p.108-109}}<ref name="Thrower"/> patented the first "syntonic" transmitter and receiver in May 1897<ref name="Patent11575">British patent GB189711575 Lodge, O. J. ''[https://worldwide.espacenet.com/publicationDetails/biblio?CC=GB&NR=189711575A&KC=A&FT=D# Improvements in Syntonized Telegraphy without Line Wires]'' filed: May 10, 1897, granted: August 10, 1898</ref><ref name="Lodge1900">Lodge's explanation of his syntonic radio system is in {{cite book
| last1= Lodge
| first1= Oliver
| title= Signaling through space without wires
| publisher= The Electrician Publishing Co.
| date= 1900
| location= London
| pages= 50–58
| url= https://archive.org/stream/cu31924012383992#page/n61/mode/2up
}}</ref><ref name="Lee1"/><ref name="Aitken1985" />{{rp|p.130–143}}
<ref name="Hong" />{{rp|p.90-93}} Lodge added an [[inductor]] (coil) between the sides of his dipole antennas, which resonated with the capacitance of the antenna to make a tuned circuit.<ref name="Thrower"/><ref name="Aitken1985"/>{{rp|p. 125-136, 254-255, 259}} Although his complicated circuit did not see much practical use, Lodge's "syntonic" patent was important because it was the first to propose a radio transmitter and receiver containing resonant circuits which were tuned to resonance with each other.<ref name="Thrower"/><ref name="Aitken1985"/>{{rp|p. 125-136, 254-255, 259}} In 1911 when the patent was renewed the Marconi Company was forced to buy it to protect its own syntonic system against infringement suits.<ref name="Aitken1985"/>{{rp|p. 125-136, 254-255, 259}}

The resonant circuit functioned analogously to a [[tuning fork]], storing oscillating electrical energy, increasing the [[Q factor]] of the circuit so the oscillations were less damped.<ref name="Aitken1985"/>{{rp|p. 125-136, 254-255, 259}} Another advantage was the frequency of the transmitter was no longer determined by the length of the antenna but by the resonant circuit, so it could easily be changed by adjustable taps on the coil. The antenna was brought into resonance with the tuned circuit using [[loading coil]]s. The energy in each spark, and thus the power output, was no longer limited by the capacitance of the antenna but by the size of the capacitor in the resonant circuit.<ref name="Sarkar" />{{rp|p.352-353, 355-358}} In order to increase the power very large capacitor banks were used. The form that the resonant circuit took in practical transmitters was the inductively-coupled circuit described in the next section.

<gallery mode="packed" heights="160">
File:Small spark gap transmitter.jpg|Demonstration inductively coupled spark transmitter 1909, with parts labeled
File:Spark gap wireless station 1910.jpg|Amateur inductively coupled spark transmitter and receiver, 1910. The spark gap is in glass bulb ''(center right)'' next to tuning coil, on top of box containing glass plate capacitor
File:Marconi spark transmitter on SS Minnetonka 1902.jpg|Standard Marconi inductively coupled transmitter on ship 1902. Spark gap is in front of induction coil, lower right. The spiral oscillation transformer is in the wooden box on the wall above the Leyden jars.
File:Leyden jars Nauen wireless telegraphy station 1908.jpg|Telefunken 25&nbsp;kW long distance transmitter built 1906 at [[Nauen Transmitter Station]], Nauen, Germany, showing large 360 Leyden jar 400&nbsp;μF capacitor bank ''(rear)'' and vertical spark gaps ''(right)''
</gallery>

====Inductive coupling====
In developing these syntonic transmitters, researchers found it impossible to achieve low damping with a single resonant circuit. A [[resonant circuit]] can only have low damping (high Q, narrow bandwidth) if it is a "closed" circuit, with no energy dissipating components.<ref name="Marconi1"/><ref name="Hong" />{{rp|p.90-93}}<ref name="Aitken1985" />{{rp|p.108-109}} But such a circuit does not produce radio waves. A resonant circuit with an antenna radiating radio waves (an "open" tuned circuit) loses energy quickly, giving it high damping (low Q, wide bandwidth). There was a fundamental tradeoff between a circuit which produced persistent oscillations which had narrow bandwidth, and one which radiated high power.<ref name="CodellaSparkRadio"/>

[[File:Inductively coupled spark gap transmitter circuit.svg|thumb|upright=1.3|Inductively coupled spark transmitter. ''C2'' is not an actual capacitor but represents the capacitance between the antenna ''A'' and ground.]]

The solution found by a number of researchers was to use two resonant circuits in the transmitter, with their coils [[inductive coupling|inductively (magnetically) coupled]], making a [[resonant transformer]] (called an ''oscillation transformer'');<ref name="CodellaSparkRadio"/><ref name="Ashley1"/><ref name="Sarkar" />{{rp|p.352-353, 355-358}} this was called an "''inductively coupled''", "''coupled circuit''"<ref name="Marriott1"/> or "''two circuit''" transmitter.<ref name="Beauchamp1"/><ref name="Jansky1"/><ref name="Hong" />{{rp|p.98-100}} See circuit diagram. The [[primary winding]] of the oscillation transformer (''L1'') with the capacitor (''C1'') and spark gap (''S'') formed a "closed" resonant circuit which generated the oscillations, while the secondary winding (''L2'') was connected to the wire antenna (''A'') and ground, forming an "open" resonant circuit with the capacitance of the antenna (''C2'').<ref name="Sarkar" />{{rp|p.352-353, 355-358}} Both circuits were tuned to the same [[resonant frequency]].<ref name="Sarkar" />{{rp|p.352-353, 355-358}} The advantage of the inductively coupled circuit was that the "loosely coupled" transformer transferred the oscillating energy of the tank circuit to the radiating antenna circuit gradually, creating long "ringing" waves.<ref name="Ashley1"/><ref name="CodellaSparkRadio"/> A second advantage was that it allowed a large primary capacitance ''(C1)'' to be used which could store a lot of energy, increasing the power output enormously.<ref name="Ashley1"/><ref name="Sarkar" />{{rp|p.352-353, 355-358}} Powerful transoceanic transmitters often had huge [[Leyden jar]] capacitor banks filling rooms ''(see pictures above)''. The receiver in most systems also used two inductively coupled circuits, with the antenna an "open" resonant circuit coupled through an oscillation transformer to a "closed" resonant circuit containing the [[detector (radio)|detector]]. A radio system with a "two circuit" (inductively coupled) transmitter and receiver was called a "four circuit" system.

The first person to use resonant circuits in a radio application was [[Nikola Tesla]], who invented the [[resonant transformer]] in 1891.<ref name="Wheeler">"''Tesla is entitled to either distinct priority or independent discovery of''" three concepts in wireless theory: "''(1) the idea of inductive coupling between the driving and the working circuits (2) the importance of tuning both circuits, i.e. the idea of an 'oscillation transformer' (3) the idea of a capacitance loaded open secondary circuit''" {{cite journal
| last= Wheeler
| first= L. P.
| title= Tesla's contribution to high frequency
| journal= Electrical Engineering
| volume= 62
| issue= 8
| pages= 355–357
| date= August 1943
| issn= 0095-9197
| doi= 10.1109/EE.1943.6435874
| s2cid= 51671246
}}</ref> At a March 1893 St. Louis lecture<ref name="Martin3">Tesla, N., "''On light and other high frequency phenomena''", in Thomas Cummerford Martin (1894) ''[https://archive.org/details/inventionsresea00teslgoog/page/n317 <!-- pg=294 --> The Inventions, Researches and Writings of Nikola Tesla, 2nd Ed.]'', p. 294-373</ref> he had demonstrated a wireless system that, although it was intended for [[wireless power transmission]], had many of the elements of later radio communication systems.<ref name="Sterling">{{cite book
| last1= Sterling
| first1= Christopher H.
| title= Biographical Encyclopedia of American Radio
| publisher= Routledge
| date= 2013
| pages= 382–383
| url= https://books.google.com/books?id=mdQq-663faAC&q=tesla&pg=PA383
| isbn= 978-1136993756
}}</ref><ref name="Uth">{{cite book
| last1= Uth
| first1= Robert
| title= Tesla, Master of Lightning
| publisher= Barnes and Noble Publishing
| date= 1999
| pages= 65–70
| url= https://books.google.com/books?id=3W6_h6XG6VAC&q=tesla&pg=PA66
| isbn= 978-0760710050
}}</ref><ref name="Sarkar" />{{rp|p.352-353, 355-358}}<ref name="Aitken1985" />{{rp|p. 125-136, 254-255, 259}}<ref name="Rockman">{{cite book
| last1= Rockman
| first1= Howard B.
| title= Intellectual Property Law for Engineers and Scientists
| publisher= John Wiley and Sons
| date= 2004
| pages= 196–199
| url= https://books.google.com/books?id=6B88P-32IIQC&pg=PA196
| isbn= 978-0471697398
}}</ref> A grounded capacitance-loaded spark-excited [[resonant transformer]] (his ''[[Tesla coil]]'') attached to an elevated wire monopole antenna transmitted radio waves, which were received across the room by a similar wire antenna attached to a receiver consisting of a second grounded resonant transformer tuned to the transmitter's frequency, which lighted a [[Geissler tube]].<ref name="Regal">{{cite book
| last1= Regal
| first1= Brian
| title= Radio: The Life Story of a Technology
| publisher= Greenwood Publishing Group
| date= 2005
| pages= 21–23
| url= https://books.google.com/books?id=N2rNO6FX8o4C&q=Tesla&pg=PA22
| isbn= 978-0313331671
}}</ref><ref name="Rockman"/><ref name="Cheney4">[https://books.google.com/books?id=HIuK7iLO9zgC&pg=PA96 Cheney, Margaret (2011) ''Tesla: Man Out Of Time'', p. 96-97]</ref> This system, patented by Tesla 2 September 1897,<ref name="Patent645576">US Patent No. 645576, Nikola Tesla, ''[https://patents.google.com/patent/US645576 System of transmission of electrical energy]'', filed: 2 September 1897; granted: 20 March 1900</ref> 4 months after Lodge's "syntonic" patent, was in effect an inductively coupled radio transmitter and receiver, the first use of the "four circuit" system claimed by Marconi in his 1900 patent ''(below)''.<ref name="Wunsch">{{cite journal
| last1= Wunsch
| first1= A. David
| title= Misreading the Supreme Court: A Puzzling Chapter in the History of Radio
| journal= Antenna
| volume= 11
| issue= 1
| date= November 1998
| url= http://www.mercurians.org/1998_Fall/misreading.htm
| access-date= 3 December 2018}}</ref><ref name="Sarkar" />{{rp|p.352-353, 355-358}}<ref name="Rockman"/><ref name="Sterling"/> However, Tesla was mainly interested in [[wireless power]] and never developed a practical radio ''communication'' system.<ref name="Coe2">{{cite book
| last1= Coe
| first1= Lewis
| title= Wireless Radio: A History
| publisher= McFarland
| date= 2006
| pages= 111–113
| url= https://books.google.com/books?id=W1JAeg1PiWIC&pg=PA112
| isbn= 978-0786426621
}}</ref><ref name="Smith">{{cite book |last=Smith |first=Craig B. |title=Lightning: Fire from the Sky|publisher=Dockside Consultants Inc. |date=2008 |url=https://books.google.com/books?id=iGZw0qPDk8YC&q=%22tesla+coil%22&pg=PT11 |isbn=978-0-615-24869-1}}</ref><ref name="Regal"/><ref name="Sarkar" />{{rp|p.352-353, 355-358}}

In addition to Tesla's system, inductively coupled radio systems were patented by [[Oliver Lodge]] in February 1898,<ref name="Patent609154">US Patent no. 609,154 Oliver Joseph Lodge, [https://patents.google.com/patent/US609154 Electric Telegraphy], filed: 1 February 1898, granted: 16 August 1898</ref><ref name="White1"/> [[Karl Ferdinand Braun]],<ref name="Hong" />{{rp|p.98-100}}<ref name="Sarkar" />{{rp|p.352-353, 355-358}}<ref name="Nahin5"/><ref name="PatentGB189922020">British patent no. 189922020 Karl Ferdinand Braun, [https://worldwide.espacenet.com/publicationDetails/biblio?DB=en.worldwide.espacenet.com&II=0&ND=3&adjacent=true&locale=en_EP&FT=D&date=19000922&CC=GB&NR=189922020A&KC=A# ''Improvements in or related to telegraphy without the use of continuous wires''], applied: 3 November 1899, complete specification: 30 June 1900, granted: 22 September 1900</ref> in November 1899, and [[John Stone Stone]] in February 1900.<ref name="Patent714756">US Patent no. 714,756, John Stone Stone [https://patents.google.com/patent/US714756 Method of electric signaling], filed: 8 February 1900, granted: 2 December 1902</ref><ref name="White1"/> Braun made the crucial discovery that low damping required "loose coupling" (reduced [[mutual inductance]]) between the primary and secondary coils.<ref name="Orton">{{cite book
| last1= Orton
| first1= John W.
| title= Semiconductors and the Information Revolution: Magic Crystals that made IT Happen
| publisher= Academic Press
| date= 2009
| pages= 37
| url= https://books.google.com/books?id=6YLL9197NfMC&q=Braun+Tesla+Marconi+coupling&pg=PA37
| isbn= 978-0080963907
}}</ref><ref name="Sarkar" />{{rp|p.352-353, 355-358}}

<gallery mode="packed" heights="170">
Image:US Patent 645576 Nikola Tesla 1897 System of transmission of electrical energy.png|Tesla's inductively coupled power transmitter ''(left)'' patented 2 September 1897<ref name="Patent645576"/>
Image:British patent 22,020-Karl Ferdinand Braun-filed 3 November 1899-fig. 2.png|Braun's inductively coupled transmitter patented 3 November 1899<ref name="PatentGB189922020"/>
Image:US Patent 714756-John Stone Stone-Method of selective electric signaling 1900 figs 5&6.png|Stone's inductively coupled transmitter ''(left)'' and receiver ''(right)'' patented 8 February 1900<ref name="Patent714756"/>
Image:Spark gap transmitter-Marconi patent 763772 fig 1.png|Marconi's inductively coupled transmitter patented 26 April 1900.<ref name="Patent763772"/>
</gallery>

Marconi at first paid little attention to syntony, but by 1900 developed a radio system incorporating features from these systems,<ref name="Orton"/><ref name="Nahin5"/> with a two circuit transmitter and two circuit receiver, with all four circuits tuned to the same frequency, using a resonant transformer he called the "jigger".<ref name="Marconi1">{{cite journal
| last1= Marconi
| first1= Guglielmo
| title= Syntonic Wireless Telegraphy
| journal= The Electrician
| date= May 24, 1901
| url= http://teslacollection.com/tesla_articles/1901/the_electrician/guglielmo_marconi/syntonic_wireless_telegraphy
| access-date= April 8, 2017}}</ref><ref name="Beauchamp1"/><ref name="Hong" />{{rp|p.98-100}} In spite of the above prior patents, Marconi in his 26 April 1900 "four circuit" or "master tuning" patent<ref name="Patent763772">British patent no. 7777, Guglielmo Marconi, [https://worldwide.espacenet.com/publicationDetails/biblio?CC=GB&NR=190007777&KC=&FT=E&locale=en_EP Improvements in apparatus for wireless telegraphy], filed: 26 April 1900, granted: 13 April 1901. Corresponding US Patent no. 763,772, Guglielmo Marconi, [https://patents.google.com/patent/US763772 Apparatus for wireless telegraphy], filed: 10 November 1900, granted: 28 June 1904.</ref> on his system claimed rights to the inductively coupled transmitter and receiver.<ref name="Sarkar" />{{rp|p.352-353, 355-358}}<ref name="White1"/><ref name="Rockman"/> This was granted a British patent, but the US patent office twice rejected his patent as lacking originality. Then in a 1904 appeal a new patent commissioner reversed the decision and granted the patent,<ref name="PBS">{{cite web
| title= Who invented radio?
| work= Tesla: Master of Lightning - companion site for 2000 PBS television documentary
| publisher= PBS.org, Public Broadcasting Service website
| date= 2000
| url= http://www.pbs.org/tesla/ll/ll_whoradio.html
| access-date= 9 April 2018}}</ref><ref name="Rockman"/> on the narrow grounds that Marconi's patent by including an antenna [[loading coil]] ''(J in circuit above)'' provided the means for tuning the four circuits to the same frequency, whereas in the Tesla and Stone patents this was done by adjusting the length of the antenna.<ref name="White1"/><ref name="Rockman"/> This patent gave Marconi a near monopoly of syntonic wireless telegraphy in England and America.<ref name="Morse3">[https://archive.org/stream/radiobeamandbroa029214mbp#page/n35/mode/2up/ Morse (1925) ''Radio: Beam and Broadcast'', p. 30]</ref><ref name="Beauchamp1"/> Tesla sued Marconi's company for patent infringement but didn't have the resources to pursue the action. In 1943 the [[US Supreme Court]] invalidated the inductive coupling claims of Marconi's patent<ref name="Findlaw">{{cite web
| title= No. 369 (1943) Marconi Wireless Co. of America v. United States
| work= United States Supreme Court decision
| publisher= Findlaw.com website
| date= June 21, 1943
| url= https://caselaw.findlaw.com/us-supreme-court/320/1.html
| access-date= March 14, 2017}}</ref> due to the prior patents of Lodge, Tesla, and Stone, but this came long after spark transmitters had become obsolete.<ref name="White1">{{cite web
| last= White
| first= Thomas H.
| title= Nikola Tesla: The Guy Who DIDN'T "Invent Radio"
| work= United States Early Radio History
| publisher= T. H. White's personal website
| date= 1 November 2012
| url= https://earlyradiohistory.us/tesla.htm
| access-date= 20 June 2018}}</ref><ref name="Rockman"/>

The inductively coupled or "syntonic" spark transmitter was the first type that could communicate at intercontinental distances, and also the first that had sufficiently narrow bandwidth that interference between transmitters was reduced to a tolerable level. It became the dominant type used during the "spark" era.<ref name="Beauchamp1"/> A drawback of the plain inductively coupled transmitter was that unless the primary and secondary coils were very loosely coupled it radiated on two frequencies.<ref name="Sarkar" />{{rp|p.352-353, 355-358}}<ref name="Beauchamp3"/> This was remedied by the quenched-spark and rotary gap transmitters'' (below)''.

In recognition of their achievements in radio, Marconi and Braun shared the 1909 [[Nobel Prize in physics]].<ref name="Sarkar" />{{rp|p.352-353, 355-358}}

===First transatlantic radio transmission===
{{multiple image
| align= right
| direction= horizontal
| header =
| image1= The Marconi Company first antenna system at Poldhu, Cornwall, December 1901.jpg
| caption1= Marconi's transmitting station at Poldhu, Cornwall, showing the original 400-wire vertical inverted cone aerial which collapsed
| width1= 225
| image2= The Marconi Company second antenna system at Poldhu, Cornwall.jpg
| caption2= The temporary antenna used in the transatlantic transmission, a fan-shaped 50-wire aerial.
| width2= 240
| footer =
}}
[[File:Marconi first transatlantic radio transmitter circuit - Poldhu 1901.png|thumb|upright=1.4|Circuit of Poldhu transmitter.<ref name="Fleming"/> Fleming's curious dual spark gap design was not used in subsequent transmitters.]]
[[File:Marconi Poldhu UK radio transmitter - made first transatlantic transmission 1901.jpg|thumb|View of Poldhu transmitter]]
Marconi decided in 1900 to attempt transatlantic communication, which would allow him to dominate Atlantic shipping and compete with [[submarine telegraph cable]]s.<ref name="Hong" />{{rp|p.60-61}}<ref name="Sarkar" />{{rp|p.387-392}} This would require a major scale-up in power, a risky gamble for his company. Up to that time his small induction coil transmitters had an input power of 100 - 200 watts, and the maximum range achieved was around 150 miles.<ref name="Hong" />{{rp|p.60-61}}<ref name="Fleming">{{cite book
| last1= Fleming
| first1= John Archibald
| title= The Principles of Electric Wave Telegraphy
| publisher= Longmans Green and Co.
| date= 1906
| location= London
| pages= 449–454
| url= https://archive.org/stream/cu31924074412739#page/n489/search/poldhu
}}</ref> To build the first high power transmitter, Marconi hired an expert in electric power engineering, Prof. [[John Ambrose Fleming]] of University College, London, who applied power engineering principles. Fleming designed a complicated inductively-coupled transmitter ''(see circuit)'' with two cascaded spark gaps ''(S1, S2)'' firing at different rates, and three resonant circuits, powered by a 25&nbsp;kW [[alternator]] ''(D)'' turned by a combustion engine.<ref name="Fleming"/><ref name="Hong" />{{rp|p.60-61}}<ref name="Belrose">{{cite conference
| last1= Belrose
| first1= John S.
| title= Fessenden and Marconi: Their differing technologies and transatlantic experiments during the first decade of this century
| conference= Proceedings of the 1995 International Conference on 100 Years of Radio
| pages= 32–34
| publisher= Institute of Engineering and Technology
| date= 5 September 1995
| location= London
| doi= 10.1049/cp:19950787
| issn= 0537-9989
| citeseerx= 10.1.1.205.7281
| s2cid= 218471926
}}</ref> The first spark gap and resonant circuit ''(S1, C1, T2)'' generated the high voltage to charge the capacitor ''(C2)'' powering the second spark gap and resonant circuit ''(S2, C2, T3)'', which generated the output.<ref name="Belrose"/> The spark rate was low, perhaps as low as 2 - 3 sparks per second.<ref name="Belrose"/> Fleming estimated the radiated power was around 10 - 12&nbsp;kW.<ref name="Fleming"/>

The transmitter was built in secrecy on the coast at [[Poldhu]], [[Cornwall]], UK.<ref name="Fleming"/><ref name="Hong" />{{rp|p.60-61}} Marconi was pressed for time because [[Nikola Tesla]] was building his own transatlantic radiotelegraphy transmitter on [[Long Island, New York]], in a bid to be first<ref name="Hong" />{{rp|p.286-288}} (this was the [[Wardenclyffe Tower]], which lost funding and was abandoned unfinished after Marconi's success). Marconi's original round 400-wire transmitting antenna collapsed in a storm 17 September 1901 and he hastily erected a temporary antenna consisting of 50 wires suspended in a fan shape from a cable between two 160 foot poles.<ref name="Fleming"/><ref name="Belrose"/><ref name="Hong" />{{rp|p.286-288}} The frequency used is not known precisely, as Marconi did not measure wavelength or frequency, but it was between 166 and 984&nbsp;kHz, probably around 500&nbsp;kHz.<ref name="Sarkar" />{{rp|p.387-392}} He received the signal on the coast of St. John's, [[Dominion of Newfoundland|Newfoundland]] using an untuned [[coherer]] [[radio receiver|receiver]] with a 400&nbsp;ft. wire antenna suspended from a [[kite]].<ref name="Sarkar" />{{rp|p.387-392}}<ref name="Fleming"/><ref name="Hong" />{{rp|p.286-288}} Marconi announced the first transatlantic radio transmission took place on 12 December 1901, from [[Poldhu]], [[Cornwall]] to [[Signal Hill, St. John's|Signal Hill, Newfoundland]], a distance of 2100&nbsp;miles (3400&nbsp;km).<ref name="Sarkar" />{{rp|p.387-392}}<ref name="Hong" />{{rp|p.286-288}}

Marconi's achievement received worldwide publicity, and was the final proof that radio was a practical communication technology. The scientific community at first doubted Marconi's report. Virtually all wireless experts besides Marconi believed that radio waves traveled in straight lines, so no one (including Marconi) understood how the waves had managed to propagate around the 300 mile high curve of the Earth between Britain and Newfoundland.<ref name="Gregersen"/> In 1902 [[Arthur Kennelly]] and [[Oliver Heaviside]] independently theorized that radio waves were reflected by a layer of [[ionization|ionized]] atoms in the upper atmosphere, enabling them to return to Earth beyond the horizon.<ref name="Gregersen"/> In 1924 [[Edward V. Appleton]] demonstrated the existence of this layer, now called the "[[Kennelly–Heaviside layer]]" or "E-layer", for which he received the 1947 [[Nobel Prize in Physics]].

Knowledgeable sources today doubt whether Marconi actually received this transmission.<ref name="Margolis">{{Cite news
| last= Margolis
| first= Laurie
| title= Faking the waves
| newspaper= The Guardian
| location= London
| date= 11 December 2001
| url= https://www.theguardian.com/education/2001/dec/11/highereducation.news
| access-date= 8 September 2018}}</ref><ref name="Belrose"/><ref name="Sarkar" />{{rp|p.387-392}} Ionospheric conditions should not have allowed the signal to be received during the daytime at that range. Marconi knew the Morse code signal to be transmitted was the letter 'S' (three dots).<ref name="Sarkar" />{{rp|p.387-392}} He and his assistant could have mistaken atmospheric [[radio noise]] ("static") in their earphones for the clicks of the transmitter.<ref name="Belrose"/><ref name="Sarkar" />{{rp|p.387-392}} Marconi made many subsequent transatlantic transmissions which clearly establish his priority, but reliable transatlantic communication was not achieved until 1907 with more powerful transmitters.<ref name="Belrose"/>

===Quenched-spark transmitters===
<gallery mode="packed" heights="140">
File:Telefunken ship radio room 1919.jpg|Ship radio room with 1.5&nbsp;kW Telefunken quenched-spark transmitter
File:Ship spark transmitter tuned circuit 1921.jpg|Tuned circuit of transmitter. ''(top)'' quenched gap, ''(center)'' oscillation transformer, Leyden jars
File:Quenched spark gap 1915.jpg|Quenched spark gap from transmitter, left. The handle turns a screw which puts pressure on the stack of cylindrical electrodes, allowing the gap widths to be adjusted.
File:Quenched spark gap - cutaway drawing.png|Cross section of portion of quenched spark gap, consisting of metal disks ''(F)'' separated by thin insulating mica washers ''(M)'' to make multiple microscopic spark gaps ''(S)'' in series
File:Powerful spark gap transmitter.png|A powerful quenched-spark transmitter in Australia. The 6 cylinders in front of the Leyden jars are the quenched spark gaps.
</gallery>

{{multiple image
| align= right
| direction= vertical
| image1= Spark-gap transmitter current waveforms.png
| caption1= Ordinary inductively coupled transmitter
| image2= Spark-gap transmitter current waveforms - quenched gap.png
| caption2= Quenched-spark transmitter<ref name="Leggett2"/>
| footer =
}}
The inductively-coupled transmitter had a more complicated output waveform than the non-syntonic transmitter, due to the interaction of the two resonant circuits. The two magnetically coupled tuned circuits acted as a [[Oscillation|coupled oscillator]], producing [[beat frequency|beats]] ''(see top graphs)''. The oscillating radio frequency energy was passed rapidly back and forth between the primary and secondary resonant circuits as long as the spark continued.<ref name="Leggett1">{{cite journal
| last1= Leggett
| first1= Bernard John
| title= Wireless Telegraphy, with special reference to the quenched-spark system
| journal= Nature
| volume= 107
| issue= 2691
| date= 1921
| pages= 51–55
| url= https://books.google.com/books?id=b843AAAAMAAJ&q=two+oscillations&pg=PA51
| doi= 10.1038/107390b0
| bibcode= 1921Natur.107..390.
| hdl= 2027/mdp.39015063598398
| s2cid= 4075587
| hdl-access= free
}}</ref><ref name="Beauchamp3"/><ref name="Huurdeman3">[https://books.google.com/books?id=SnjGRDVIUL4C&pg=PA271&dq=Wien+telefunken+squenched Huurdeman, Anton (2003) ''The Worldwide History of Telecommunications'', p. 271-272]. This author misspells the word "quenched" as "squenched"</ref> Each time the energy returned to the primary, some was lost as heat in the spark.<ref name="Huurdeman3"/><ref name="Beauchamp3"/> In addition, unless the coupling was very loose the oscillations caused the transmitter to transmit on two separate frequencies.<ref name="Beauchamp3"/><ref name="Burns2"/> Since the narrow passband of the receiver's resonant circuit could only be tuned to one of these frequencies, the power radiated at the other frequency was wasted.

This troublesome backflow of energy to the primary circuit could be prevented by extinguishing (quenching) the spark at the right instant, after all the energy from the capacitors was transferred to the antenna circuit.<ref name="Leggett2">[https://archive.org/details/wirelesstelegra00legggoog/page/n76 <!-- pg=55 --> Bernard Leggett (1921) ''Wireless Telegraphy, with special reference to the quenched-spark system'', p. 55-59]</ref><ref name="Burns2">{{cite book
| last1= Burns
| first1= Russell W.
| title= Communications: An International History of the Formative Years
| publisher= Institute of Electrical Engineers
| date= 2004
| pages= 361–362
| url= https://books.google.com/books?id=7eUUy8-VvwoC&q=quenched&pg=PA361
| isbn= 978-0863413278
}}</ref> Inventors tried various methods to accomplish this, such as air blasts and [[Elihu Thomson]]'s [[magnetic blowout]].<ref name="Beauchamp3"/><ref name="Burns2"/>

In 1906, a new type of spark gap was developed by German physicist [[Max Wien]],<ref name="Bard">{{cite book
| last1= Bard
| first1= Allen J.
| last2= Inzelt
| first2= György
| last3= Scholz
| first3= Fritz
| title= Electrochemical Dictionary
| edition= 2nd
| publisher= Springer Science and Business Media
| date= 2012
| pages= 972
| url= https://books.google.com/books?id=4TBWg3dIyKQC&q=max+wien&pg=PA972
| isbn= 978-3642295515
}}</ref> called the ''series'' or ''quenched'' gap.<ref name="Stanley1">{{cite book
| last1= Rupert
| first1= Stanley
| title= Text-book on Wireless Telegraphy, Vol. 1: General Theory and Practice
| publisher= Longmans Green and Co.
| date= 1919
| location= London
| pages= 200–204
| url= https://books.google.com/books?id=b4tCAQAAMAAJ&q=quenched+spark+singing&pg=PA200
}}</ref><ref name="Beauchamp2">{{cite book
| last1= Beauchamp
| first1= Ken
| title= History of Telegraphy
| publisher= IET
| date= 2001
| pages= 194–197
| url= https://books.google.com/books?id=k3XCkncd83AC&pg=PA193
| isbn= 978-0852967928
}}</ref><ref name="Leggett3">[https://archive.org/details/wirelesstelegra00legggoog/page/n81 <!-- pg=60 --> Bernard Leggett (1921) ''Wireless Telegraphy, with special reference to the quenched-spark system'', p. 60-63]</ref><ref name="Huurdeman3"/> A quenched gap consisted of a stack of wide cylindrical electrodes separated by thin insulating spacer rings to create many narrow spark gaps in series,<ref name="Beauchamp2"/> of around {{convert|0.1|-|0.3|mm|inch|sigfig=1|abbr=on}}.<ref name="Stanley1"/> The wide surface area of the electrodes terminated the ionization in the gap quickly by cooling it after the current stopped. In the inductively coupled transmitter, the narrow gaps extinguished ("quenched") the spark at the first nodal point (<span style="color:red;">Q</span>) when the primary current momentarily went to zero after all the energy had been transferred to the secondary winding ''(see lower graph)''.<ref name="Leggett2"/> Since without the spark no current could flow in the primary circuit, this effectively uncoupled the secondary from the primary circuit, allowing the secondary resonant circuit and antenna to oscillate completely free of the primary circuit after that (until the next spark). This produced output power centered on a single frequency instead of two frequencies. It also eliminated most of the energy loss in the spark, producing very lightly damped, long "ringing" waves, with decrements of only 0.08 to 0.25<ref name="Arco">{{cite journal
| last1= von Arco
| first1= Georg
| title= The New Telefunken Telegraph: A combination of the arc and spark systems
| journal= Scientific American Supplement
| volume= 67
| issue= 1746
| pages= 390
| date= 19 June 1909
| url= https://books.google.com/books?id=FgYiAQAAMAAJ&q=Max+wein+%22quenched+arc%22+damping&pg=PA390
| doi =10.1038/scientificamerican06191909-390supp
| access-date= 5 December 2018}}</ref> (a Q of 12-38) and consequently a very "pure", narrow bandwidth radio signal. Another advantage was the rapid quenching allowed the time between sparks to be reduced, allowing higher spark rates of around 1000&nbsp;Hz to be used, which had a musical tone in the receiver which penetrated radio static better. The quenched gap transmitter was called the "singing spark" system.<ref name="Arco"/><ref name="Stanley1"/>

The German wireless giant [[Telefunken]] Co., Marconi's rival, acquired the patent rights and used the quenched spark gap in their transmitters.<ref name="Leggett3"/><ref name="Stanley1"/><ref name="Huurdeman3"/>
{{Clear}}

===Rotary gap transmitters===
A second type of spark gap that had a similar quenching effect<ref name="Hyder" /> was the "rotary gap", invented by Tesla in 1896<ref name="Patent20981">British patent GB189620981 Henry Harris Lake for Nikola Tesla ''[https://worldwide.espacenet.com/publicationDetails/biblio?II=6&ND=3&adjacent=true&locale=en_EP&FT=D&date=18961121&CC=GB&NR=189620981A&KC=A# Improvements relating to the production, regulation, and utilization of electric currents of high frequency, and apparatus therefore]'' filed: 22 September 1896, granted: 21 November 1896</ref><ref name="Morse1">{{cite book
| last1= Morse
| first1= A. H.
| title= Radio: Beam and Broadcast
| publisher= Ernst Benn, Ltd.
| date= 1925
| location= London
| pages= 25, 138–148
| url= https://archive.org/stream/radiobeamandbroa029214mbp#page/n29
}}</ref> and applied to radio transmitters by [[Reginald Fessenden]] and others.<ref name="Sarkar" />{{rp|p.359–362}}<ref name="Beauchamp3">{{cite book
| last1= Beauchamp
| first1= Ken
| title= History of Telegraphy
| publisher= IET
| date= 2001
| pages= 192–194
| url= https://books.google.com/books?id=k3XCkncd83AC&pg=PA193
| isbn= 978-0852967928
}}</ref> It consisted of multiple electrodes equally spaced around a disk rotor spun at high speed by a motor, which created sparks as they passed by a stationary electrode.<ref name="CodellaSparkRadio"/><ref name="Jansky1"/> By using the correct motor speed, the rapidly separating electrodes extinguished the spark after the energy had been transferred to the secondary.<ref name="Hyder" /><ref name="CodellaSparkRadio"/><ref name="Sarkar" />{{rp|p.359–362}}<ref name="Beauchamp3"/> The rotating wheel also kept the electrodes cooler, important in high-power transmitters.

<gallery mode="packed" heights="130">
File:Rotary spark gap 1919.jpg|A typical rotary spark gap used in low-power transmitters
File:Rotary spark gap transmitter.jpg|Small rotary spark transmitter, 1918
File:Murdock 1 kW rotary gap spark transmitter 1914.jpg|1 kilowatt rotary spark transmitter, 1914.
File:Fessenden synchronous spark transmitter.jpg|[[Reginald Fessenden|Fessenden]]'s 35&nbsp;kW synchronous rotary spark transmitter, built 1905 at Brant Rock, Massachusetts, with which he achieved the first 2 way transatlantic communication in 1906 on 88&nbsp;kHz.
File:Navy NAA spark transmitter Arlington 1913.jpg|US Navy 100&nbsp;kW rotary gap transmitter built by Fessenden in 1913 at Arlington, Virginia. It transmitted on 113&nbsp;kHz to Europe, and broadcast the US's first radio time signal.
</gallery>

There were two types of rotary spark transmitter:<ref name="Hyder" /><ref name="Sarkar" />{{rp|p.359–362}}<ref name="CodellaSparkRadio"/><ref name="Beauchamp3"/><ref name="Belrose"/>
*''Nonsynchronous'': In the earlier rotary gaps, the motor was not synchronized with the frequency of the AC transformer, so the spark occurred at random times in the AC cycle of the voltage applied to the capacitor. The problem with this was the interval between the sparks was not constant.<ref name="Sarkar" />{{rp|p.359–362}} The voltage on the capacitor when a moving electrode approached the stationary electrode varied randomly between zero and the peak AC voltage. The exact time when the spark started varied depending on the gap length the spark could jump, which depended on the voltage. The resulting random phase variation of successive damped waves resulted in a signal that had a "hissing" or "rasping" sound in the receiver.<ref name="Kennedy"/>
*''Synchronous'': In this type, invented by Fessenden around 1904, the rotor was turned by a [[synchronous motor]] in synchronism with the cycles of the AC voltage to the transformer, so the spark occurred at the same points of the voltage sine wave each cycle. Usually it was designed so there was one spark each half cycle, adjusted so the spark occurred at the peak voltage when the capacitor was fully charged.<ref name="Kennedy"/> Thus the spark had a steady frequency equal to a multiple of the AC line frequency, which created [[harmonic (music)|harmonics]] with the line frequency. The synchronous gap was said to produce a more musical, easily heard tone in the receiver, which cut through interference better.<ref name="Kennedy"/>
To reduce interference caused by the "noisy" signals of the burgeoning numbers of spark transmitters, the 1912 US Congress "Act to Regulate Radio Communication" required that "''the logarithmic decrement per oscillation in the wave trains emitted by the transmitter shall not exceed two tenths''"<ref name="Jansky1"/><ref name="CodellaSparkRadio"/><ref name="RadioAct1912">{{cite web
| title= An act to regulate radio communication
| work= Public 264 S. 6412 approved 13 August 1912
| publisher= United States Congress
| date= 1912
| url= https://babel.hathitrust.org/cgi/pt?id=uc1.b3420424;view=1up;seq=8
|pages = 6–14
| access-date= 14 April 2019}} included in ''Radio Communication Laws of the United States'', July 27, 1914 edition, Department of Commerce, United States government printing office</ref> (this is equivalent to a [[Q factor]] of 15 or greater). Virtually the only spark transmitters which could satisfy this condition were the quenched-spark and rotary gap types above,<ref name="Jansky1"/> and they dominated wireless telegraphy for the rest of the spark era.
{{Clear}}

==== Marconi's timed spark system ====
In 1912 in his high-power stations Marconi developed a refinement of the rotary discharger called the "timed spark" system, which generated what was probably the nearest to a [[continuous wave]] that sparks could produce.<ref name="Bucher2">{{cite book
| last1= Bucher
| first1= Elmer E.
| title= Practical Wireless Telegraphy
| publisher= Wireless Press, Inc.
| date= 1917
| location= New York
| pages= 274–275
| url= https://archive.org/stream/wirelesstele00buchrich#page/274/mode/2up
}}</ref><ref name="Coursey">{{cite journal
| last1= Coursey
| first1= Phillip R.
| title= The Marconi Timed-Spark Continuous-Wave Transmitter
| journal= Wireless World
| volume= 7
| issue= 78
| pages= 310–316
| date= September 1919
| url= https://www.americanradiohistory.com/Archive-Wireless-World/10s/Wireless-World-1919-09.pdf
| access-date= 19 August 2018}}</ref><ref name="Sarkar" />{{rp|p.399}} He used several identical resonant circuits in parallel, with the capacitors charged by a DC [[dynamo]].<ref name="Goldsmith">{{cite book
| last1= Goldsmith
| first1= Alfred N.
| title= Radio Telephony
| publisher= Wireless Press, Inc.
| date= 1918
| location= New York
| pages= 73–75
| url= https://archive.org/stream/radiotelephony00goldgoog#page/n86/mode/2up
}}</ref> These were discharged sequentially by multiple rotary discharger wheels on the same shaft to create overlapping damped waves shifted progressively in time, which were added together in the oscillation transformer so the output was a [[superposition principle|superposition]] of damped waves. The speed of the discharger wheel was controlled so that the time between sparks was equal to an integer multiple of the wave period. Therefore, oscillations of the successive wave trains were [[in phase]] and reinforced each other. The result was essentially a continuous sinusoidal wave, whose amplitude varied with a ripple at the spark rate. This system was necessary to give Marconi's transoceanic stations a narrow enough bandwidth that they didn't interfere with other transmitters on the narrow [[very low frequency|VLF]] band. Timed spark transmitters achieved the longest transmission range of any spark transmitters, but these behemoths represented the end of spark technology.<ref name="Sarkar" />{{rp|p.399}}

{{multiple image
| align= center
| direction= horizontal
| header =
| image1= Marconi transatlantic wireless station at Carnarvon, Wales 1917.jpg
| width1= 302
| caption1= Transmitter building, showing the 36 feedlines feeding power to the 3,600 ft. flattop wire antenna.
| image2= Marconi Carnarvon 300kW transatlantic spark transmitter 1919 - tuning coil.png
| width2= 235
| caption2= 5 ft diameter primary coil of oscillation transformer, consisting of 3 turns of specialized [[litz wire]] one foot thick
| image3= Marconi Carnarvon 300kW transatlantic spark transmitter 1919 - rotary spark dischargers.png
| width3= 260
| caption3= The three 5 ft rotary spark discharger wheels of the "timed spark" system.
| footer= Marconi 300&nbsp;kW transatlantic timed spark transmitter built 1916 at [[Caernarfon|Carnarvon]], [[Wales]], one of the most powerful spark transmitters ever built. During World War I it transmitted telegram traffic at 200 words per minute on 21.5&nbsp;kHz to receivers in Belmar, New Jersey.<ref name="WirelessWorld">{{cite journal
| title= Great Wireless Stations: Carnarvon
| journal= Wireless World
| volume= 7
| issue= 78
| pages= 301–307
| date= September 1919
| url= https://www.americanradiohistory.com/Archive-Wireless-World/10s/Wireless-World-1919-09.pdf
| access-date= 19 August 2018}}</ref> The roar of the spark could reportedly be heard a kilometer away. On 22 September 1918 it transmitted the first wireless message from Britain to Australia, a distance of 15,200&nbsp;km (9,439&nbsp;miles).<ref name="MacKinnon">{{cite web
| last= MacKinnon
| first= Colin
| title= The first direct wireless messages from England to Australia
| work= Australian Amateur Radio History
| publisher= VK2DYM's military radio and radar information site
| date= 2004
| url= https://www.qsl.net/vk2dym/radio/Marconi.htm
| access-date= 4 May 2018}}</ref> In 1921 it was replaced by [[Alexanderson alternator]] transmitters.
}}

===The "spark" era===
The first application of radio was on ships, to keep in touch with shore, and send out a distress call if the ship were sinking.<ref name="White3">{{cite web
| last= White
| first= Thomas H.
| title= Section 12: Radio at Sea (1891-1922)
| work= United States Early Radio History
| publisher= T. H. White's personal website
| date= 2003
| url= https://earlyradiohistory.us/sec005.htm
| access-date= 2 October 2018}}</ref> The Marconi Company built a string of shore stations and in 1904 established the first Morse code distress call, the letters ''CQD'', used until the Second International Radiotelegraphic Convention in 1906 at which ''[[SOS]]'' was agreed on. The first significant marine rescue due to radiotelegraphy was the 23 January 1909 sinking of the luxury liner [[RMS Republic (1903)|RMS ''Republic'']], in which 1500 people were saved.

{| style="background:#e0e0e0;" align="right" border="1" cellpadding="5" cellspacing="0"
! colspan="4" style="background:#c0c0c0; text-align:center;" |Radio frequencies used by spark transmitters during the wireless telegraphy era<ref name="Moorcroft">{{cite book |last1= Moorcroft |first1= John Harold |last2= Pinto |first2= A. |last3= Curry |first3= Walter Andrew |title= Principles of Radio Communication |publisher= John Wiley and Sons |date= 1921 |pages= [https://archive.org/details/principlesradio03currgoog/page/n373 357] |url= https://archive.org/details/principlesradio03currgoog }}</ref>
|-
! style="background:#d0d0d0; text-align:center;" | Uses
! style="background:#d0d0d0; text-align:center;" border="1" | Frequency<br/>(kilohertz)
! style="background:#d0d0d0; text-align:center;" border="1" | Wavelength<br/>(meters)
! style="background:#d0d0d0; text-align:center;" | Typical power<br/> range (kW)
|-
| valign="top" | Amateur || > 1500 || < 200 || 0.25 - 0.5
|-
| Ships || 500, 660, 1000 || 600, 450, 300 || 1 - 10
|-
| Navy || 187.5 - 500 || 1600 - 600 || 5 - 20
|-
| Moderate size land stations || 187.5 - 333 || 1600 - 900 || 5 - 20
|-
| Transoceanic stations || 15 - 187.5 || 20,000 - 1600 || 20 - 500
|}

Spark transmitters and the [[crystal radio|crystal receivers]] used to receive them were simple enough that they were widely built by hobbyists.<ref name="Hyder" /> During the first decades of the 20th century this exciting new high tech hobby attracted a growing community of "[[radio amateur]]s", many of them teenage boys, who used their homebuilt sets recreationally to contact distant amateurs and chat with them by Morse code, and relay messages.<ref name="CodellaSqueakBox">{{cite web
| last= Codella
| first= Christopher F.
| title= The Squeak Box
| work= Ham Radio History
| publisher= Codella's private website
| date= 2016
| url= http://w2pa.net/HRH/the-squeak-box/
| access-date= 22 May 2018}}</ref><ref name="White2">{{cite web
| last= White
| first= Thomas H.
| title= Section 12: Pioneering Amateurs (1900-1912)
| work= United States Early Radio History
| publisher= earlyradiohistory.us
| date= 2003
| url= https://earlyradiohistory.us/sec012.htm
| access-date= 26 June 2018}}</ref> Low-power amateur transmitters ("squeak boxes") were often built with "[[trembler coil|trembler]]" [[ignition coil]]s from early automobiles such as the [[Ford Model T]].<ref name="CodellaSqueakBox"/> In the US prior to 1912 there was no government regulation of radio, and a chaotic "wild west" atmosphere prevailed, with stations transmitting without regard to other stations on their frequency, and deliberately interfering with each other.<ref name="White2" /><ref name="Howeth2">{{cite book
| last1= Howeth
| first1= L. S.
| title= The History of Communications - Electronics in the U.S. Navy
| publisher= U.S. Navy
| date= 1963
| pages= 69, 117
| url= https://archive.org/stream/historyofcommuni00howe#page/116
}}</ref><ref name="CodellaFirstRegulation"/> The expanding numbers of non-syntonic broadband spark transmitters created uncontrolled congestion in the airwaves, interfering with commercial and military wireless stations.<ref name="CodellaFirstRegulation">{{cite web
| last= Codella
| first= Christopher F.
| title= The first regulations
| work= Ham Radio History
| publisher= Codella's private website
| date= 2016
| url= http://w2pa.net/HRH/the-first-regulations/
| access-date= 22 May 2018}}</ref>

The {{RMS|Titanic}} sinking 14 April 1912 increased public appreciation for the role of radio, but the loss of life brought attention to the disorganized state of the new radio industry,<ref name="SanFranciscoCall">{{cite news
  | title = To Check Wireless Anarchy
  | newspaper = San Francisco Call
  | location = San Francisco, California
  | pages = 22
  | language = 
  | publisher = Charles Shortridge
  | date = 7 July 1912
  | url = https://chroniclingamerica.loc.gov/lccn/sn85066387/1912-07-07/ed-1/seq-22/
  | access-date = 5 February 2024}}</ref> and prompted regulation which corrected some abuses.<ref name="White2"/> Although the ''Titanic'' radio operator's ''CQD'' distress calls summoned the {{RMS|Carpathia}} which rescued 705 survivors, the rescue operation was delayed four hours because the nearest ship, the [[SS Californian|SS ''Californian'']], only a few miles away, did not hear the ''Titanic''{{'}}s call as its radio operator had gone to bed. This was held responsible for most of the 1500 deaths.  Existing international regulations required all ships with more than 50 passengers to carry wireless equipment, but after the disaster subsequent regulations mandated ships have enough radio officers so that a round-the-clock radio watch could be kept.  US President Taft and the public heard reports of chaos on the air the night of the disaster, with amateur stations interfering with official naval messages and passing false information.<ref name="SanFranciscoCall" /><ref name="NewYorkHerald">{{cite news
  | title = President Moves to Stop Mob Rule of Wireless
  | newspaper = New York Herald
  | location = New York, NY
  | pages = 2
  | language = 
  | publisher = James Gordon Bennett, Jr.
  | date = 17 April 1912
  | url = https://fultonhistory.com/Newspaper%2014/New%20York%20NY%20Herald/New%20York%20NY%20Herald%201912/New%20York%20NY%20Herald%201912%20-%203562.pdf
  | access-date = 5 February 2024}}</ref>  In response Congress passed the 1912 Radio Act, in which licenses were required for all radio transmitters, maximum damping of transmitters was limited to a decrement of 0.2 to get old noisy non-syntonic transmitters off the air, and amateurs were mainly restricted to the unused frequencies above 1.5&nbsp;MHz and output power of 1 kilowatt.<ref name="RadioAct1912"/><ref name="CodellaFirstRegulation"/><ref name="Hyder" />

[[File:Quenched spark transmitter - Nauen Radio Station, Nauen Germany 1921.jpg|thumb|Telefunken 100&nbsp;kW transoceanic quenched spark transmitter at [[Nauen Transmitter Station]], [[Nauen]], Germany was the most powerful radio transmitter in the world when it was built in 1911]]

The largest spark transmitters were powerful transoceanic radiotelegraphy stations with input power of 100 - 300&nbsp;kW.<ref name="Pickworth">{{cite journal
| last1= Pickworth
| first1= George
| title= Marconi´s 200kW transatlantic transmitter
| journal= Electronics World
| volume= 102
| issue= 1718
| date= January 1994
| url= http://hjem.get2net.dk/helthansen/marconi_tx.htm
| archive-url = https://archive.today/20021020055257/http://hjem.get2net.dk/helthansen/marconi_tx.htm
| archive-date = 2002-10-20
| url-status = dead
| access-date= 22 March 2018}}</ref><ref name="Bucher1">{{cite book
| last1= Bucher
| first1= Elmer E.
| title= Practical Wireless Telegraphy
| publisher= Wireless Press, Inc.
| date= 1917
| location= New York
| pages= 288–307
| url= https://archive.org/stream/wirelesstele00buchrich#page/302/mode/2up
}}</ref> Beginning about 1910, industrial countries built global networks of these stations to exchange commercial and diplomatic [[telegram]] traffic with other countries and communicate with their overseas colonies.<ref name="Leggett4">{{cite journal
| last1= Leggett
| first1= Bernard John
| title= Wireless Telegraphy, with special reference to the quenched-spark system
| journal= Nature
| volume= 107
| issue= 2691
| date= 1921
| pages= 299–305
| url= https://books.google.com/books?id=b843AAAAMAAJ&pg=PA299
| doi= 10.1038/107390b0
| bibcode= 1921Natur.107..390.
| hdl= 2027/mdp.39015063598398
| s2cid= 4075587
| hdl-access= free
}}</ref><ref name="Lescarboura">{{cite book
| last1= Lescarboura
| first1= Austin C.
| title= Radio for Everybody
| publisher= Scientific American Publishing Co.
| date= 1922
| pages= [https://archive.org/details/RadioForEverybody/page/n279 259]–263
| url= https://archive.org/details/RadioForEverybody
}}</ref><ref name="Headrick">{{cite book
| last1= Headrick
| first1= Daniel R.
| title= The Tentacles of Progress: Technology Transfer in the Age of Imperialism, 1850-1940
| publisher= Oxford University Press
| date= 1988
| location= New York
| pages= 126–130
| url= https://books.google.com/books?id=0wjDxYlArQAC&pg=PA126
| isbn= 978-0198021780
}}</ref> During [[World War I]], radio became a strategic defensive technology, as it was realized a nation without long distance radiotelegraph stations could be isolated by an enemy cutting its [[submarine telegraph cable]]s.<ref name="Lescarboura" />   Most of these networks were built by the two giant wireless corporations of the age: the British [[Marconi Wireless Telegraph Co|Marconi Company]], which constructed the [[Imperial Wireless Chain]] to link the possessions of the [[British Empire]], and the German [[Telefunken]] Co. which was dominant outside the British Empire.<ref name="Leggett4"/> Marconi transmitters used the timed spark rotary discharger, while Telefunken transmitters used its quenched spark gap technology. [[punched paper tape|Paper tape machines]] were used to transmit Morse code text at high speed. To achieve a maximum range of around 3000 – 6000 miles, transoceanic stations transmitted mainly in the [[very low frequency]] (VLF) band, from 50&nbsp;kHz to as low as 15 – 20&nbsp;kHz. At these wavelengths even the largest antennas were [[electrically short]], a tiny fraction of a wavelength tall, and so had low [[radiation resistance]] (often below 1 ohm), so these transmitters required enormous wire [[umbrella antenna|umbrella]] and [[T-antenna|flattop]] antennas up to several miles long with large capacitive toploads, to achieve adequate efficiency. The antenna required a large [[loading coil]] at the base, 6 – 10 feet tall, to make it resonant with the transmitter.

===Continuous waves===
Although their damping had been reduced as much as possible, spark transmitters still produced [[Damped wave (radio transmission)|damped wave]]s, which due to their large bandwidth caused interference between transmitters.<ref name="Terman"/><ref name="Aitken1985" />{{rp|p.72-79}} The spark also made a very loud noise when operating, produced corrosive [[ozone]] gas, eroded the spark electrodes, and could be a fire hazard.<ref name="Hyder" />  Despite its drawbacks, most wireless experts believed along with Marconi that the impulsive "whipcrack" of a spark was necessary to produce radio waves that would communicate long distances.<ref name="Sarkar"/>{{rp|p.374}}<ref name="McNicol" />{{rp|p.78}}

From the beginning, physicists knew that another type of waveform, [[continuous wave|continuous]] [[sinusoidal]] waves (CW), had theoretical advantages over damped waves for radio transmission.<ref name="Fitzgerald">George Fitzgerald as early as 1892 described a spark oscillator as similar to the oscillations produced when a cork pops out of a winebottle, and said what was needed was a continuous electromagnetic "whistle".  He realized that if the resistance of a tuned circuit were made zero or negative it would produce continuous oscillations, and tried to make an electronic oscillator by exciting a tuned circuit with negative resistance from a dynamo, what would today be called a parametric oscillator, but was unsuccessful.  G. Fitzgerald, ''On the Driving of Electromagnetic Vibrations by Electromagnetic and Electrostatic Engines'', read at the January 22, 1892 meeting of the Physical Society of London, in {{cite book
 |last        = Larmor
 |first       = Joseph, Ed.
 |title       = The Scientific Writings of the late George Francis Fitzgerald
 |publisher   = Longmans, Green and Co.
 |date        = 1902
 |location    = London
 |pages       = 277–281
 |url         = https://books.google.com/books?id=G0bPAAAAMAAJ&pg=PA277
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20140707134922/https://books.google.com/books?id=G0bPAAAAMAAJ&pg=PA277
 |archive-date = 2014-07-07
}}</ref><ref name="Aitken2014" />{{rp|p.4–7, 32–33}} Because their energy is essentially concentrated at a single frequency, in addition to causing almost no interference to other transmitters on adjacent frequencies, continuous wave transmitters could transmit longer distances with a given output power.<ref name="Aitken1985" />{{rp|p.72-79}} They could also be [[modulation|modulated]] with an [[audio signal]] to carry sound.<ref name="Aitken1985" />{{rp|p.72-79}} The problem was no techniques were known for generating them. The efforts described above to reduce the damping of spark transmitters can be seen as attempts to make their output approach closer to the ideal of a continuous wave, but spark transmitters could not produce true continuous waves.<ref name="Aitken2014" />{{rp|p.4–7, 32–33}}

Beginning about 1904, continuous wave transmitters were developed using new principles, which competed with spark transmitters. Continuous waves were first generated by two short-lived technologies:<ref name="Aitken1985" />{{rp|p.72-79}}
*The [[arc converter]] (Poulsen arc) transmitter, invented by [[Valdemar Poulsen]] in 1904 used the [[negative resistance]] of a continuous [[electric arc]] in a [[hydrogen]] atmosphere to excite oscillations in a [[resonant circuit]].
*The [[Alexanderson alternator]] transmitter, developed between 1906 and 1915 by [[Reginald Fessenden]] and [[Ernst Alexanderson]], was a huge rotating alternating current generator ([[alternator]]) driven by an electric motor at a high enough speed that it produced [[radio frequency]] current in the [[very low frequency]] range.
These transmitters, which could produce power outputs of up to one [[megawatt]], slowly replaced the spark transmitter in high-power radiotelegraphy stations. However spark transmitters remained popular in two way communication stations because most continuous wave transmitters were not capable of a mode called "break in" or "listen in" operation.<ref name="Handy">{{cite book
  | last = Handy
  | first = Frances Edward
  | title = The Radio Amateur's Handbook, 1st Ed.
  | publisher = American Radio Relay league
  | date = 1926
  | location = Hartford, CN
  | pages = 123–124
  | language = 
  | url = https://www.worldradiohistory.com/BOOKSHELF-ARH/Technology/ARRL/The-Radio-Amateur's-Handbook-ARRL-1926-1st.pdf
  | archive-url= 
  | archive-date=
  | doi = 
  | id = 
  | isbn = 
  | mr = 
  | zbl = 
  | jfm =}}</ref>  With a spark transmitter, when the telegraph key was up between Morse symbols the carrier wave was turned off and the receiver was turned on, so the operator could listen for an incoming message. This allowed the receiving station, or a third station, to interrupt or "break in" to an ongoing transmission.  In contrast, these early CW transmitters had to operate continuously; the [[carrier wave]] was not turned off between Morse code symbols, words, or sentences but just detuned, so a local [[radio receiver|receiver]] could not operate as long as the transmitter was powered up. Therefore, these stations could not receive messages until the transmitter was turned off.

===Obsolescence===
[[Image:Marconi Model P-4 2 kW ship spark transmitter.jpg|thumb|upright= 1.0|Marconi 2 kilowatt ship spark transmitter, 1920. ]]
All these early technologies were superseded by the [[vacuum tube]] [[positive feedback|feedback]] [[electronic oscillator]], invented in 1912 by [[Edwin Armstrong]] and [[Alexander Meissner]], which used the [[triode]] [[vacuum tube]] invented in 1906 by [[Lee de Forest]].<ref name="Hempstead"/> Vacuum tube oscillators were a far cheaper source of continuous waves, and could be easily [[modulated]] to carry sound. Due to the development of the first high-power transmitting tubes by the end of World War I, in the 1920s tube transmitters replaced the arc converter and alternator transmitters, as well as the last of the old noisy spark transmitters.

The 1927 International Radiotelegraph Convention in Washington, D.C. saw a political battle to finally eliminate spark radio.<ref name="Schroeder">{{cite book
| last1= Schroeder
| first1= Peter B.
| title= Contact at Sea: A History of Maritime Radio Communications
| publisher= The Gregg Press
| date= 1967
| pages= 26–30
| url= https://books.google.com/books?id=sEeaJC_y22EC&q=spark&pg=PA14-IA15
}}</ref> Spark transmitters were obsolete at this point, and [[radio broadcasting|broadcast radio]] audiences and aviation authorities were complaining of the disruption to radio reception that noisy legacy marine spark transmitters were causing. But shipping interests vigorously fought a blanket prohibition on damped waves, due to the capital expenditure that would be required to replace spark equipment that was still being used on older ships. The Convention prohibited licensing of new land spark transmitters after 1929.<ref name="Howeth3">{{cite book
| last1= Howeth
| first1= L. S.
| title= The History of Communications - Electronics in the U.S. Navy
| publisher= U.S. Navy
| date= 1963
| pages= [https://archive.org/details/historyofcommuni00howe/page/509 509]
| url= https://archive.org/details/historyofcommuni00howe
| isbn= 978-1365493225
}}</ref> Damped wave radio emission, called Class B, was banned after 1934 except for emergency use on ships.<ref name="FCC"/><ref name="Howeth3"/> This loophole allowed shipowners to avoid replacing spark transmitters, which were kept as emergency backup transmitters on ships through World War II.

===Legacy===
One legacy of spark-gap transmitters is that [[radio operator]]s were regularly nicknamed "Sparky" long after the devices ceased to be used. Even today, the German verb ''funken'', literally, "to spark", also means "to send a radio message".

The spark gap oscillator was also used in nonradio applications, continuing long after it became obsolete in radio. In the form of the [[Tesla coil]] and [[Oudin coil]] it was used until the 1940s in the medical field of [[diathermy]] for deep body heating.<ref name="Strong">{{cite book
| last1= Strong
| first1= Frederick Finch
| title= High Frequency Currents
| publisher= Rebman Co.
| date= 1908
| location= New York
| url= https://archive.org/details/highfrequencycu00strogoog
| page= [https://archive.org/details/highfrequencycu00strogoog/page/n65 41]
}}</ref><ref name="Kovács">{{cite book
| last1= Kovács
| first1= Richard
| title= Electrotherapy and Light Therapy
| edition= 5th
| publisher= Lea and Febiger
| date= 1945
| location= Philadelphia
| pages= 187–188, 197–200
| url= https://archive.org/stream/electrotherapyli00kovrich#page/186/mode/2up
}}</ref> High oscillating voltages of hundreds of thousands of volts at frequencies of 0.1 - 1&nbsp;MHz from a Tesla coil were applied directly to the patient's body. The treatment was not painful, because currents in the radio frequency range do not cause the physiological reaction of [[electric shock]]. In 1926 [[William T. Bovie]] discovered that RF currents applied to a scalpel could cut and cauterize tissue in medical operations, and spark oscillators were used as [[electrosurgery]] generators or "Bovies" as late as the 1980s.<ref name="Carr">{{cite journal
| last1= Carr
| first1= Joseph J.
| title= Early radio transmitters
| journal= Popular Electronics
| volume= 7
| issue= 5
| pages= 43–46
| date= May 1990
| url= https://www.americanradiohistory.com/Archive-Poptronics/90s/90/PE-1990-03.pdf
| access-date= 21 March 2018}}</ref>

In the 1950s a Japanese toy company, Matsudaya, produced a line of cheap [[remote control]] toy trucks, boats and robots called Radicon, which used a low-power spark transmitter in the controller as an inexpensive way to produce the radio control signals.<ref name="Parker">{{cite web
| last= Parker
| first= John
| title= Flotsam & Jetsam – Control by Radio
| work= Model Boats website
| publisher= MyTimeMedia Ltd., UK
| date= September 2017
| url= http://www.modelboats.co.uk/news/article/flotsam-jetsam---control-by-radio/25527
| access-date= 20 March 2018}}</ref><ref name="Findlay">{{cite journal
| last1= Findlay
| first1= David A.
| title= Radio Controlled Toys Use Spark Gap
| journal= Electronics
| volume= 30
| issue= 9
| pages= 190
| date= September 1, 1957
| url= http://www.americanradiohistory.com/Archive-Electronics/50s/Electronics-1957-09-01.pdf
| access-date= November 11, 2015}}</ref> The signals were received in the toy by a [[coherer]] receiver.

Spark gap oscillators are still used to generate high-frequency high voltage needed to initiate welding arcs in [[gas tungsten arc welding]].<ref name="LincolnElectric">{{cite web
| title= TIG Welding Series: The Power to Perform
| publisher= Lincoln Electric website
| date= 2006
| url= http://www.lincolnelectric.com/knowledge/articles/content/tigseriesreliable.asp
| access-date= 6 January 2019
| quote= ...the number one maintenance item on a TIG machine is cleaning and adjusting the spark gap.
| archive-date= 16 May 2006
| archive-url= https://web.archive.org/web/20060516040041/http://www.lincolnelectric.com/knowledge/articles/content/tigseriesreliable.asp
| url-status= dead
}}</ref> Powerful spark gap pulse generators are still used to simulate [[electromagnetic pulse|EMPs]].

==See also==
*[[History of radio]]
*[[Invention of radio]]
*[[Amateur radio]]
*[[Antique radio]]
*[[Coherer]]
*[[Crystal radio]]

==References==
{{Reflist}}

==Further reading==
*{{cite book |last=Morecroft |first=John Harold |title=Principles of Radio Communication |chapter-url=https://archive.org/details/principlesofradi00moreuoft |chapter=Spark Telegraphy |access-date=September 12, 2015 |year=1921 |publisher=Wiley |location=New York |pages=275–363}}
*{{cite book |last=Zenneck |first=Jonathan |others=Translated by Alfred E. Seelig |author-link=Jonathan Zenneck |title=Wireless Telegraphy |url=https://archive.org/details/wirelesstelegra00zenngoog |access-date=September 14, 2015 |year=1915 |publisher=McGraw-Hill Book Company |location=New York }}

==External links==
{{Commons category|Spark-gap transmitters}}
*[http://home.freeuk.net/dunckx/wireless/wireless.html Alternator, Arc and Spark]
*[https://www.ewh.ieee.org/reg/7/millennium/radio/radio_radioscientist.html Fessenden and the Early History of Radio Science]
*[https://web.archive.org/web/20130325082927/http://www.vistech.net/users/w1fji/spark.html Brief history of spark]
*[https://newsm.org/buildings/massie/ Massie Spark Transmitter] The new England Wireless and Steam Museum
*{{cite web|url=http://www.physics.otago.ac.nz/ursi/belrose/spark.html|title=The Sounds of a Spark Transmitter with audio|archive-url=https://web.archive.org/web/20110718231452/http://www.physics.otago.ac.nz/ursi/belrose/spark.html|archive-date=July 18, 2011}}
*[http://www.radioana.com/ The Sparks Telegraph Key Review]
*[http://www.geojohn.org/Radios/MyRadios/Sparks/Sparks.html Radio Technology in common use circa 1914]
*[https://www.electronics-notes.com/articles/history/spark-gap-transmitters/history.php Spark gap transmitter history & operation]

{{Telecommunications}}
{{Authority control}}

[[Category:History of radio technology]]
[[Category:Radio electronics]]
[[Category:Electric arcs]]
[[Category:Electric power conversion]]
[[Category:Telegraphy]]