Vacuum tubes are electronic components used to amplify, switch, modify and/or create analog signals by controlling the movement of electrons in low-pressure spaces. These components are used in electronics and can also be referred to as electron tubes in the United States, and thermionic valves or valves in other parts of the world, especially Britain.
Although many uses for vacuum tubes have been phased out, they were heavily used in early generations of electronic devices such as radios, televisions, and early computers. Vacuum tubes are still used in specialized engineering applications such as high-power radio frequency transmitters, audio amplification, and in some display devices.
Vacuum tubes act as insulted, heat-resistant, tubular envelopes that surround a vacuum containing electrodes. When electrical contacts are placed on the ends of the vacuum tube, the current is directed to flow through the vacuum via an airtight seal. Most vacuum tubes, leads are designed to plug into a tube socket for easy replacement. When in operation, vacuum types typically contain two heat sources: a filament or heater and a source at the anode. Filament or heater sources may contain either directly or indirectly heated cathodes. Directly heated cathodes for vacuum tubes are similar to an incandescent lamp and may glow brightly like such, but most glow dimly. Bright emitting tubes contain a tungsten filament, which is alloyed with thorium, which allows it to emit sufficient electrons. Dull emitting vacuum tubes also contain the tungsten filament, but are coated with strontium, calcium, barium oxides to emit at a lower temperature. Vacuum tubes containing indirectly heated cathodes often consist of a nickel tube and the same outer coating of strontium, calcium, barium oxides. These cathodes are fitted with tungsten filament within the tube to heat it. The tungsten within indirectly heated cathodes is often uncoiled and coated in an aluminum oxide to insult it from the nickel tube of the cathode. Vacuum tubes are also heated at the anode, where electrons are accelerated by the applied voltage to strike the anode and impart a considerable amount of its energy to raise the temperature.