Closeup of the filament on 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, often thermionic power generation pdf resulting in lamp failure. One of the bulbs with which Edison discovered thermionic emission.
Electrons released by the filament were attracted to the plate when it had a positive voltage. Thermionic emission is the thermally induced flow of charge carriers from a surface or over a potential-energy barrier. This occurs because the thermal energy given to the 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.
The hot cathode can be a metal filament, a coated metal filament, or a separate structure of metal or carbides or borides of transition metals. The term “thermionic emission” is now also used to refer to any thermally-excited charge emission process, even when the charge is emitted from one solid-state region into another. The magnitude of the charge flow increases dramatically with increasing temperature.
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