Vacuum tubes, based on thermal emission, were the basis for electronics and communications until the transition to solid-state technologies in the late 20th century.
Vacuum tubes are based on a filament wire that heats the cathode, which emits electrons. The electrons are attracted to a positive voltage on the anode. Additional electrodes between the cathode and the anode are called grids, and control the current between the cathode and the anode.
The most common tubes were Diode (which was used as a rectifier or detector), Triode (with one grid), Tetrode (two grids), and Pentode (three grids).
Vacuum Tubes
