Thermionic Valve
A thermionic valve is the British name for what Americans call a vacuum tube, and the adjective points to the physical principle that makes it work: thermionic emission, the release of electrons from a metal heated hot enough that some of them gain the energy to escape its surface. Where the vacuum-tube entry describes the device and its uses, this entry concerns the physics of the emission and the lineage of the name, from an unexplained glow-and-current curiosity in Edison's lamps in the 1880s to the equation that describes it in quantitative terms.
The effect was observed long before it was understood. In 1880 Thomas Edison noticed that in an evacuated light bulb with an extra metal plate, current could flow from the glowing filament to the plate but not the other way, and he patented a related indicator in 1883; the phenomenon became known as the Edison effect. Earlier observations of electricity near hot bodies were made by Frederick Guthrie in 1873. Nobody could explain why until J. J. Thomson identified the electron in 1897, after which the effect could be read as a hot filament boiling electrons into the surrounding vacuum, a picture that Owen Willans Richardson developed quantitatively in the early 1900s.
Richardson's law gives the physics in one expression. Metals contain conduction electrons bound by an energy barrier called the work function, typically a few electron-volts, with tungsten near 4.5 electron-volts. Heating does not lower the barrier; it lets a tiny tail of the thermal energy distribution climb over it. The emitted current density is proportional to the square of the absolute temperature multiplied by an exponential of minus the work function divided by Boltzmann's constant times temperature, J = A × T² × exp(−W / kT). The exponential makes emission rise explosively with temperature, which is why cathodes must run hot, and why lowering W matters. Richardson received the 1928 Nobel Prize in Physics for his work on thermionic phenomena and this law, which is also called the Richardson-Dushman equation.
The name records a design choice and a lineage. Fleming, who had studied Edison-effect lamps and worked as a consultant in the lighting and telegraph industries, built in 1904 a lamp-like device with a hot filament and a plate and called it an oscillation valve, after the way a mechanical valve lets a fluid pass in one direction only. In Britain the word valve stuck for decades, giving thermionic valve and wireless valve, whereas American usage settled on tube. The term thermionic itself combines heat and ion, an echo of an era before the electron was understood, which is why it can mislead: the emitted particles are electrons, not ions.
The practical engineering followed from the equation. Because current depends exponentially on W, cathode designers sought coatings that lower the work function. Pure tungsten wire must be run white-hot at roughly 2,500 K, thoriated tungsten emits at lower temperature, and the oxide-coated cathodes used in most receiving valves run far cooler, closer to 1,000 K, saving heater power. Emission also competes with the cloud of electrons already in front of the cathode, the space charge, which limits the current at a given plate voltage, a behavior summarized by the Child-Langmuir law. These two effects explain the shape of every valve's characteristic curves.
Thermionic emission remains current, even where valves have disappeared. Electron guns in scanning electron microscopes, X-ray tubes and some electron-beam machines still rely on hot cathodes. The same mathematics returns in solid-state devices: in a Schottky diode current crosses a barrier by thermionic emission, and in a modern MOSFET the sub-threshold current, which sets a minimum of about 60 millivolts of gate swing per tenfold current change at room temperature, is governed by electrons climbing a barrier thermally. A common mistake is to treat it as a purely historical topic or to equate thermionic emission with field emission, which is quantum tunneling at a cold surface under a strong electric field.
Read the thermionic valve as the name for Fleming's one-way heated-cathode device, and thermionic emission as the physics beneath it: electrons climbing the work-function barrier with a probability that rises sharply with temperature, a rule that still applies to barriers in chips.