An active device in which conduction between two electrodes takes place in an envelope that is sealed or continuously exhausted and contains a gas or a vacuum. Tubes that are evacuated to a high vacuum are termed hard tubes; those that contain traces of a gas are soft tubes. The concentration of gaseous atoms in soft tubes is sufficient to cause some modification to the characteristics compared with those of hard tubes. Electron tubes frequently contain more than two electrodes. See also vacuum tube; thermionic valve.