English: The dynatron tube invented by Albert W. Hull in 1918, a special purpose vacuum tube with negative resistance due to a phenomenon called secondary emission. It was the first vacuum tube that had negative resistance. It had some limited use as an oscillator in the 1920s, but did not prove practical.

The dynatron consisted of an evacuated glass envelope with three electrodes, organized something like a triode. Between the heated filament and cylindrical plate electrodes there is a perforated screen similar to a grid, called the anode. Unlike in an ordinary triode, the anode is operated at a higher voltage than the plate. Electrons released by the hot filament due to thermionic emission are attracted to the positively charged plate and strike it. Due to their velocity they knock electrons out of the plate, called secondary electrons. Since the anode is at a higher potential than the plate, the secondary electrons are attracted to it. This represents a current away from the plate, reducing the net plate current. As the voltage on the plate is increased, more secondary electrons are produced, so the plate current decreases. This is called negative differential resistance. If a tuned circuit is connected between the plate and filament, the negative resistance of the tube will cancel the positive resistance of the tuned circuit, resulting in a circuit with zero AC resistance. Spontaneous oscillations will be excited in the tuned circuit, at its resonant frequency