MOTIONS OF ELECTRONS 185 



whereby charges are transferred between two plates or 

 electrodes which are maintained at different potentials. 

 We have not as yet discussed the manner in which 

 electrons may be released at the negative electrode. 

 We have seen that in the case of metals the electrons 

 are comparatively loosely held in the atomic or molecu- 

 lar structure with the result that they are always avail- 

 able for conduction. Free electrons, which may migrate 

 within a metal body from one point to another, are the 

 cause of the better electrical conductivity of metals 

 and also of their better heat conductivity. Those 

 substances which are good conductors of electricity are 

 also usually efficient in transferring molecular energy 

 from a point of high temperature to one of low tem- 

 perature. 



Increased temperature of a metal results in increased 

 energy on the part of these free electrons, as we should 

 expect from the idea of the equipartition of energy 

 which was developed in the preceding chapter. From 

 the standpoint of the kinetic theory we are not, there- 

 fore, surprised to learn that there comes a time, as the 

 temperature of a metal is increased, when some of the 

 electrons have acquired a sufficient kinetic energy to 

 carry them beyond the influence of the molecules of 

 the metal J itself. In other words, we can picture to 

 ourselves a phenomenon of the boiling of the electrons 

 of a metal quite similar to the boiling of a liquid. 



Because the electrons are of smaller mass than the 

 molecules we should expect a very pronounced boiling 

 of electrons long before the temperature of the metal 



1 The phenomenon is essentially similar to that of surface 

 tension, which is discussed on p. 221. 



