INTERACTIONS OF MOVING ELECTRONS 211 



ductors is less. If we allow them to tractate we find 

 the currents reduced, but when we attempt to restore 

 them to then- former value we have to supply a larger 

 amount of energy than was derived in external work. 

 In restoring the currents to then* former value we have 

 made the same number of electrons per second pass 

 through a cross section of the conductor. If this 

 represents the same kinetic energy as before it should 

 have been necessary to supply to the system only an 

 amount of energy equal to the external work. We 

 have to supply as much again. 1 It therefore appears 

 that the same number of electrons moving with the 

 same velocity now possess a greater kinetic energy. 

 This conclusion we must reach if we accept the prin- 

 ciple of the conservation of energy. 



We are accustomed to consider the kinetic energy as 

 mv^/2. Can the mass of the electron be different in 

 the two cases? This brings us squarely against the 

 question as to what we mean by mass. We have been 

 using mass as meaning amount of matter, but we have 

 had occasion to deal either with electrically neutral 

 bodies or with bodies, like the pith balls mentioned hi 

 Chapter VII, which are composed of millions and 

 millions of neutral molecules and comparatively few 

 excess electrons. We have measured masses in terms 

 of the standard kilogram by a comparison of the 

 gravitational potential energies of the unknown and 

 known mass. Or, by our own muscular sense, we have 

 compared inertias and said that they were equal when 

 equal forces produced equal accelerations. We have then 

 said that the bodies of equal inertia had equal masses. 

 1 Cf. footnote of p. 215. 



