THERMIONICS. Thermionics is the branch of science which deals with the influence of heat on matter in generating atomic or sub-atomic electrically charged particles (ions or elec trons) and with the electrical properties of the assemblages so produced. Effects of this general character have been familiar to physicists for about two hundred years but the subject at tracted little attention until about the beginning of the present century when J. J. Thomson discovered that the carriers of these discharges at very low gas pressures were electrons and 0. W. Richardson discovered the fundamental law connecting the phe nomena with temperature. In recent years interest in the subject has extended to a still wider field owing to the great importance of its electrotechnical applications, particularly to the art of electrical communication. (See art. THERMIONIC VALVE.) In the treatment which follows, attention will be confined mainly to the emission of electrons by solids owing to the practical and theoretical interest of this aspect of the subject.
If a solid body, for example a wire, is heated in a vacuous enclosure it is found in general to emit both positively and nega tively electrified particles ; as may be ascertained by having a neighbouring insulated electrode in the enclosure and connecting it outside through a battery of suitable polarity and a galvanom eter or other detector of electric current. If the wire is of a refractory material such as tungsten or platinum and the heat ing is continued the positive emission, which is caused by electro positive contaminants such as potassium, dies away; so that ultimately in a highly evacuated enclosure the emission consists solely of electrons, that is to say sub-atomic negatively charged particles. In this way it comes about that a heated body admits of the escape of negative electricity from its surface, but not of positive. Thus an evacuated enclosure containing a hot and a cold conductor only allows the passage of electricity in one direc tion, the negative electricity passing from the hot to the cold body, and so functions as an electrical valve. This emission varies very rapidly with the temperature in a manner which will now be investigated. The electron currents from the surfaces of bodies obtained in this way at high temperatures may be very con siderable.
This emission of electrons from hot bodies is a special kind of evaporation. It differs from the evaporation of a solid chiefly in the fact that the particles in the gaseous phase are electrically charged. Thus a hot body in an evacuated enclosure will be surrounded by an electron gas which will be capable of exerting a pressure and in fact will have the same dynamical properties as other gases having the same temperature and molecular con centration. The reason why the electrons do not all prefer to leave the solid for the enclosure is that they have to do a certain amount of work in passing through its surface. This electron
atmosphere will be capable of being in equilibrium with all the bodies present in the enclosure at any fixed temperature and in this condition each body will emit in a given time as many electrons as it receives.
If we suppose a single hot body to be contained in such an enclosure maintained at the absolute temperature T°K and if the enclosure is provided with a movable piston we can obtain use ful work from the pressure p exerted by the electron atmosphere on the piston. If there are n electrons in unit volume, if v is the volume of the enclosure and ci) is the change of energy when one electron passes from the hot body to the surrounding space, the increment dS in the entropy of the system due to an infinitesimal displacement of the piston is where II is Planck's constant and a is the proportion of an incident beam of electron gas reflected at the surface.
The most usual device for testing the implications of equation (8) is to use an electrically heated wire placed on the axis of a surrounding cylinder, both these elements being suitably mounted in an evacuated container of glass or quartz. If a steady poten tial difference is maintained between the hot wire and the cylinder by means of an external battery, the current across the gap can be measured by a galvanometer placed in series with the battery. If the wire is held at a high positive potential no current will flow, but with a small positive potential it is found that a small current flows in a direction contrary to the applied potential dif ference. The reason for this is that the large potential differences are high enough to stop all the emitted electrons from reaching the opposite electrode. The electrons, however, are not emitted with zero velocity in general but some of them have sufficient initial kinetic energy to carry them to the opposite electrode against a small opposing electric field. If the wire is charged negatively, then the number of electrons which are carried across the gap increases as the accelerating field is increased. This increase does not go on without limit, but a stage is soon reached when the electron current becomes independent of any further increase in the applied potential differences. The reason for this is that the field is now so strong that it carries to the cold elec trode all the electrons which are emitted from the hot wire. This limiting value of the current is known as the saturation current and when reduced to unit area it is the quantity to which the symbol in equation (8) applies. The chief reason why this cur rent rises to the saturation value gradually and that any accel erating potential, however small, is insufficient to guide the elec trons across the gap is the effect of the self repulsion of the elec trons. This effect is more marked the higher the temperature, that is to say the greater the maximum current to be dealt with.