352 RICHARDSON— DYNAMICAL EFFECTS OF [Apni 22, 



electrons. In this wa}^ the time which is required for electricity to be 

 transferred will be greater so that the specific electrical conductivity 

 will be diminished. This is the explanation of the second effect. 

 Both these effects are complicated by the action of the electrons on 

 the atoms so that the foregoing description is only to be regarded 

 as a rough outline of what really occurs. 



So far we have only considered the wa}' in which the electron 

 theory of conduction explains a number of phenomena which were 

 familiar before it was enunciated. The power to do this is a neces- 

 sary attribute of every scientific theory. A scientific theory, how- 

 ever, is often much more useful than this in that it leads to the pre- 

 diction of phenomena which would hardly have been foreseen without 

 its aid. The present theory has been able to prove its usefulness in 

 this way. as the principles underlying it have enabled us to develop 

 a new chapter in physical science, a chapter to which I have ventured 

 to give the name of Thermionics. Thermionics relates to the emis- 

 sion of electrified particles by hot bodies and the phenomena to which 

 they give rise. 



It is found that all bodies when heated to a sufficiently high tem- 

 perature give rise to an emission of both negatively and positively 

 charged particles. In many ways the negative emission is the more 

 interesting as the particles emitted are negative electrons having prop- 

 erties identical with those of the carriers of the cathode rays. The 

 connection between this emission of negative electrons and the trans- 

 portation of electricity in a metallic conductor is very intimate. We 

 have seen that, in order to explain the phenomena exhibited by me- 

 tallic conduction, it is necessary to suppose that such conductors 

 contain large numbers of "free" electrons. If these electrons are 

 maving about freely inside the conductor, as we have supposed, the 

 question at once arises as to why they do not escape into the sur- 

 rounding atmosphere. It is clear that they do not do so, otherwise 

 there would be a leakage of electricity from the surface of all con- 

 ductors at ordinary temperatures. The answer must be that there 

 are forces at the surface of the metal which are sufficiently great to 

 prevent them from escaping. Now consider what we should ex- 

 pect to happen as the temperature of such a body is raised. We have 

 supposed that the average kinetic energy of the contained electrons 



