THERMIONIC VALVE (Thermionic Tube, Audion or Radiotron). The term thermionic valve is applied to any form of electric discharge tube in which one or more of the elec trodes is heated so as emit elec tricity by the process of thermi onic emission. More commonly the term indicates a highly-ex hausted tube in which the electric current consists of negative elec trons evaporated from an electri cally heated wire filament and collected by a metal sheet elec trode, known as the plate or anode, which surrounds the fila ment. The maximum current which can pass through such a tube is equal to the electron cur rent emitted by the filament or cathode. If the current passing through the tube is not as large as this emitted current, as is usu ally the case when the tube is in operation, the difference is made up by the return of electrons to the filament. The current reach ing the anode, called the anode current, may be caused to vary by varying the potential of the anode with respect to the filament, as in a two-electrode valve or diode; or the anode potential may be fixed and the anode current controlled electrically by means of a perforated electrode or grid situated between the filament and anode, as in a three-electrode valve or triode. Tubes with more than three electrodes are used, but their behaviour, in general, is very similar to that of a triode.
for wireless transmitting stations, however, are operated at high anode voltages and pure tungsten filaments are chiefly used in them because of their greater rugged ness and ability to withstand accidental voltage overloads.
The admixture of less than 1% of thoria to a tung sten filament is found to increase very appreciably the elec tron evaporation at any given temperature. According to I. Langmuir the thoria is turned to metallic thorium when the wire is heated and a thorium layer one molecule thick forms on the tungsten surface. It is this layer which reduces the amount of work necessary to transfer an electron across the surface and which thus increases the electron emission. A thoriated tungsten filament is usually run at 2,000° K. and since, at that temperature it is not very bright, it is called a dull-emitter. At that tem perature the electricity it emits is many times that emitted by a tungsten filament run at 2,400° K. with the same energy supply. A typical example of a filament with a chemical coating is the oxide-coated platinum filament. This is also a dull-emitter and is run at a temperature as low as 1,200° K. or even less. It is usually a platinum ribbon coated with a mixture of barium and strontium oxides and carbonates. The pronounced thermionic emis sion appears to be due to the presence of a film of metallic barium on the filament surface. Another way of obtaining such a barium film is to use a filament with a copper coating on which is put barium azide (Ba2N6). When such a filament is heated the nitrogen is driven off leaving the barium which alloys with the copper and thus remains on the surface of the filament.
The relation between the electron emission per sq.cm. (l) of filament surface and the temperature (T) may be expressed by 0. W. Richardson's formula The ratio of the total emission from a filament to the rate of supply of energy to heat is a constant for a given temperature. For the operating temperatures used in present-day valves this ratio is about 0.003 amp. per watt for tungsten, 0.050 amp. per watt for thoriated tungsten and 0.130 amp. per watt for oxide coated platinum.