Electrovacuum devices

In 1904 John Ambrose Fleming invented the diode, similar to electrical light device with a metal plate inside. When the diode is connected to the circuit, through it in one direction only passes a small current from plate to filament. Thus, the diode can rectify an alternating current, that is, to generate from it unidirectional.
In 1906, the American Lee De forest found that by placing between the filament and the plate in the diode wire lattice, it is possible to control the current that passes through it. A small voltage change on the grid causes a significant change in the current between the filament and the plate, that is, the vacuum tube can amplify signals.