Thermionic Valves

Electron tubes in which electrons are emitted from a heated electrode (cathode) . The British physicist John Ambrose Fleming developed the first valve in 1904 for use in radio receivers. It was a two electrode valve or diode, containing a cathode and an anode in an evacuated glass bulb. Two years later the American Lee De Forest produced a triode, which had a third electrode (grid) between the cathode and the anode. In the triode, a small change of voltage on the grid has a great effect on the electron flow, or current passing between cathode and anode. in this way signals can be amplified. Thermionic valves with further grids are available for different uses. For many applications these days, however, thermionic valves have been replaced by semiconductor devices, such as Transistors