MOTIONS OF ELECTRONS 179 



gravitational potential it is only necessary that it be" 

 released. In its free fall it acquires kinetic energy which 

 is available upon impact for conversion into other 

 forms of energy. The larger portion of this energy is, 

 of course, converted into heat, that is, the haphazard 

 motion of the molecules of the partners to the collision. 

 Some of the energy is, however, transmitted away 

 through space in the form of a sound wave, the 

 mechanics of which we have previously considered. 



This case of free fall is one which is possible for 

 electrons only under rather special conditions. We ob- 

 tain it in the laboratory by using an evacuated vessel 

 containing two metal plates which we connect to the 

 positive and negative plates of the battery. 



In all other cases with which we have to do the 

 electrons are impeded in their fall. This is true when- 

 ever a " conductor " connects the two points of dif- 

 ferent potential. If the fall is impeded the energy is 

 subtracted during it instead of entirely at its end. The 

 difference between the two cases is much like that of a 

 ball which is either dropped from the roof of a building 

 or allowed to roll down the stairs. In the latter case 

 its descent consists of a large number of free falls 

 through short distances. It never, therefore, acquires 

 the high velocity which is obtained in the other case. 

 With each impact the kinetic energy acquired since 

 the previous impact is converted into heat. This is 

 essentially the phenomenon of the conduction of elec- 

 tricity through metallic conductors. 



When the fall of electrons is not free but takes place 

 through a gaseous medium an impact with a gaseous 

 molecule may be sufficient to shake loose or otherwise 



