Mullard E88CC

In electronics, a vacuum tube, electron tube (in North America), thermionic valve, tube or valve is a device controlling electric current through a vacuum in a sealed container. The container is often thin transparent glass in a roughly cylindrical shape. Within the glass is a vacuum with a cathode and anode at either end. Electrons flow from the cathode to the anode through the vacuum, a demonstration of the Edison Effect. The introduction of a grid between the cathode and anode makes it possible to amplify a current. A weak current applied to the grid will be amplified by the current traveling through the vacuum and will be available at one of the tubes electrodes.

Vacuum tubes are used for rectification, amplification, switching, or similar processing or creation of electrical signals. Tubes rely on thermionic emission of electrons from a hot filament or hot cathode. Electrons travel to the anode (or plate) when it is at a positive voltage with respect to the cathode. Additional electrodes between the cathode and anode regulate current, allowing a tube to amplify or to switch.

Tubes were critical to the development of electronic technology, which drove the expansion and commercialization of radio broadcasting, television, radar, sound reinforcement, sound recording and reproduction, large telephone networks, analog and digital computers, and industrial process control. Although some applications had counterparts using earlier technologies such as the spark gap transmitter or mechanical computers, it was the invention of the triode vacuum tube and its capability of electronic amplification that made these technologies widespread and practical.