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A thermionic valve or vacuum tube is an amplifying device consisting of a glass tube containing several electrodes in a vacuum. An electrically heated [[Glossary:Cathode|cathode]] emits [[Glossary:Electron|electrons]] which are attracted by an [[Glossary:Anode|anode]]. Their flow is controlled by the [[Glossary:Volt|voltage]] on a wire mesh [[Glossary:Grid|grid]] in their path. Long since superseded by much smaller and less power-hungry [[Glossary:Transistor|transistors]] in all but a few niche applications.