Vacuum tube
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A vacuum tube, electron tube, valve, or tube, is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.
Later thermionic vacuum tubes, mostly miniature style, some with top cap connections for higher voltages
Hot tubes in an audio power amplifier, emitting their distinctive red-orange glow
Illustration representing a primitive triode vacuum tube and the polarities of the typical DC operating potentials. Not shown are the impedances (resistors or inductors) that would be included in series with the C and B voltage sources.
Radio station signal generator with vacuum tubes
Thermionic emission
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Thermionic emission is the liberation of electrons from an electrode by virtue of its temperature. This occurs because the thermal energy given to the charge carrier overcomes the work function of the material. The charge carriers can be electrons or ions, and in older literature are sometimes referred to as thermions. After emission, a charge that is equal in magnitude and opposite in sign to the total charge emitted is initially left behind in the emitting region. But if the emitter is connected to a battery, the charge left behind is neutralized by charge supplied by the battery as the emitted charge carriers move away from the emitter, and finally the emitter will be in the same state as it was before emission.
Closeup of the filament in a low pressure mercury gas-discharge lamp showing white thermionic emission mix coating on the central portion of the coil. Typically made of a mixture of barium, strontium and calcium oxides, the coating is sputtered away through normal use, eventually resulting in lamp failure.
One of the bulbs with which Edison discovered thermionic emission. It consists of an evacuated glass light bulb containing a carbon filament (hairpin shape), with an additional metal plate attached to wires emerging from the base. Electrons released by the filament were attracted to the plate when it had a positive voltage.
Schottky-emitter electron source of an electron microscope