thermoionic valves An electronic device, based on thermoionic emission, that has the ability to control the flow of current. An evacuated glass tube contains the cathode, which is either directly heated (tungsten) or indirectly heated (oxide coated). The diode is the simplest thermoionic valve, consisting of the cathode that emits the electrons, and the anode (or plate). The anode is kept at positive potential with respect to the cathode and collects the electrons emitted by the cathode. The current flows when the plate is at positive potential with respect to the cathode. This unidirectional nature of the diode can be used for rectification *.

A third element to the diode was added by Lee De forest (in 1907), between the plate and the cathode. The third element, called the control grid, provides the ability to the valve to amplify signals. This led to the sensational development of radio communication, broadcasting and electronics. The control grid is a wire mesh surrounding the cathode. Its position is nearer to the cathode than the plate. A potential placed on control grid has much larger effect on plate current than same potential on plate. This permits amplification of input signal.

Tetrode was further improvement of the triode, where another grid called the screen grid is inserted between the control grid and plate. This eliminates the feedback of energy cause by the grid plate capacitance.