Kinds of Filaments
Author: J.B. Hoag
The larger tubes, such as those that are used in powerful broadcasting stations and in X-ray outfits, have a filament made of tungsten wire. Although this is not the most efficient emitter of electrons it has a sufficient ruggedness to warrant its use in these large tubes. For the smaller tubes, on the other hand, the filaments can be coated with special materials which permit them to emit electrons more copiously at a given temperature.
These special coatings are of such a nature that the electrons can escape from the underlying metal with greater ease than in their absence. This could be compared to the spreading of some kind of film over the surface of water which would permit the water underneath to evaporate more rapidly at a given temperature. The scientific measure of the goodness of the filament, insofar as its electron emitting power is concerned, is called the work function, defined as the number of volts to remove one electrostatic unit of electricity from the metal. The smaller the value of the work function, the more copious the emission of electrons, other conditions being the same. Conversely, the same number of electrons can be obtained from a filament whose surface has been properly treated when it is operated at a lower temperature than from an untreated surface at a higher temperature. Hence, "thoriated" and "coated" filaments are often spoken of as dull emitters. Dull-emitter filaments have low values of the work function, of the order of 1 or 2 volts, in contrast with the pure metals whose work function is of the order of 3 to 6 volts. As another comparison between the various types of filaments let us examine the following numbers: a pure tungsten filament heated to 2,000° K. will deliver 1 milliampere of electron current for 23 units (watts) of input heating energy (per sq. cm. of surface); a thoriated filament will give 350 milliamperes for 13 watts per sq. cm. at 1,500° K.; an oxide-coated filament will give about 150 milliamperes for 6 watts input at 1,200° K. These are 0.04, 27, and 25 milliamperes per sq. cm. per watt, respectively, but are only approximate figures, and other factors, such as life and durability against vibration, are to be considered in practical cases.