English: The 100 kW spark-gap transmitter at the US Navy's first radio communication station, call sign NAA, constructed in 1913 at Arlington, Virginia. It was a rotary spark gap set invented by Reginald Fessenden and built by his firm National Electric Signaling Co. (NESCO) and was used until 1925. It transmitted by radiotelegraphy on 113 kHz (2650 meters) at ranges exceeding 6000 miles. It was used to communicate with Europe and Navy radio stations on the East and West coasts, and in the US possessions of the Canal Zone (Panama) and Puerto Rico. It also broadcast the first US radio time signal at noon and 10 o'clock each night, provided by the nearby US Naval Observatory, which was received all over the US and used to set clocks in factories, offices, and jeweler's stores.

It consisted of a 200 HP, 300 RPM, 220V, 3 phase synchronous motor (not visible), driving by belt a 1250 RPM, 500 Hz alternator (far left), whose output was increased to high voltage with a step-up transformer which charged a bank of high pressure air capacitors (lower right). The motor also turned the water-cooled synchronous rotary spark gap (left) which produced 500 sparks per second. This audio frequency cut through interference better. The spark gap discharged the capacitors through the primary of the oscillation transformer (top center), creating radio frequency damped waves. The secondary of the oscillation transformer was connected to a huge multiwire flattop wire antenna supported on one 600 ft and two 450 ft masts, through antenna loading coils (top right) which matched the impedance of the antenna to the transmitter. Information from Kreisinger, Robert (1980). ""NAA" - Arlington". Sparks Journal 3 (3): 1, 6-10. Society of Wireless Pioneers, Inc.. Retrieved on February 3, 2015..