English: The circuit of a dynatron oscillator, a type of vacuum tube electronic oscillator which uses negative resistance caused by secondary emission of electrons from the anode of a tetrode tube. The screen grid is biased at a higher voltage than the anode, causing secondary electrons knocked out of the plate by electrons from the cathode to flow to the screen. Therefore the anode current decreases with an increase in cathode-to-anode voltage. The negative differential resistance of the tube cancels the inherent positive resistance of the tuned circuit LC connected across the anode, creating in effect a tuned circuit with zero AC resistance. Spontaneous oscillations arise at the tuned circuit's resonant frequency ${\displaystyle \scriptstyle f\;=\;2\pi /{\sqrt {(}}LC)}$ started by noise in the circuit when it is turned on. Invented in 1918 by Albert Hull and used from the 1920s to the 1940s the dynatron oscillator is now obsolete.