An active device in which conduction between two electrodes takes place in an envelope that is sealed and evacuated to a sufficiently low pressure that its electrical characteristics are independent of any residual gas. Tubes frequently contain additional electrodes to modify the electrical behaviour. The term vacuum tube, originally American, is applied in particular to the thermionic valve. Such devices, largely displaced by their solid-state equivalents, are still used for example for high-frequency high-power applications such as radio communications.