page_title: Thermionic emission
hierarchical_section_title: Thermionic emission
caption_attribution_description: English: A replica of one of the bulbs with which Thomas Alva Edison discovered the Edison Effect (thermionic emission) in the early 1880s. Made by Clayton H. Sharp in 1921 as an experiment to see if Edison's bulb could be used like a Fleming valve as a detector in a radio receiver to rectify radio waves. It worked. It consists of an Edison incandescent light bulb, an evacuated glass bulb with a hairpin shaped bamboo carbon filament, with an additional platinum plate (visible between the arms of the filament) attached to wires emerging from the base of the bulb. A current through the filament heated it white-hot. Edison discovered that the hot filament emitted negatively-charged particles (later descovered to be electrons) an effect that was called the Edison effect. He demonstrated this by applying a separate voltage between the filament and plate. When the plate had a positive voltage, the electrons were attracted to it and a current flowed through the tube from filament to plate. When the plate had a negative voltage, the electrons were repelled so no current flowed through the tube. Edison found no practical use for this effect, but in 1904 John Ambrose Fleming invented a similar tube called the Fleming valve which was used to rectify radio signals in the first radio receivers, which evolved into the diode vacuum tube. Alterations to image: partially removed aliasing artifacts (crosshatched lines in background) due to scanning of halftone photo using FFT filter in GIMP.
context_page_description: 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.
  The classical example of thermionic emission is that of electrons from a hot cathode into a vacuum in a vacuum tube. 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. Vacuum emission from metals tends to become significant only for temperatures over 1,000 K.
context_section_description: Thermionic emission is the liberation of electrons from an electrode by virtue of its temperature (releasing of energy supplied by heat). 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.
  The classical example of thermionic emission is that of electrons from a hot cathode into a vacuum (also known as thermal electron emission or the Edison effect) in a vacuum tube. 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. Vacuum emission from metals tends to become significant only for temperatures over 1,000 K (730 °C; 1,340 °F).
  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. This process is crucially important in the operation of a variety of electronic devices and can be used for electricity generation (such as thermionic converters and electrodynamic tethers) or cooling. The magnitude of the charge flow increases dramatically with increasing temperature.
caption: 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.
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