A vacuum tube containing a filament heated by low-voltage current and emitting extremely small negatively charged particles of electricity (electrons). In a welding circuit, the electronic tube functions as a single-pole contactor, making and breaking circuits. Small electronic tubes are used as relays and contactors in timing and control circuits; large tubes are used as power contactors.
Electronic tubes have largely been replaced by transistors and other solid-state devices; however, some vacuum tube equipment is still in use.
