212 THE REALITIES OF MODERN SCIENCE 



The only conclusion at which we can arrive is that 

 part or all of the mass of an electron is what we might 

 call " electromagnetic" to distinguish it from the sort 

 of masses with which we have so far had to deal. 

 If we say that all of the mass of an electron is of this 

 character, that is, not invariable in its inertia, then we 

 are in accord with the modern theory. 



Consider now the conditions of the conductors, which 

 we were discussing, in order to determine what factors 

 influence the effective inertia of an electron. We 

 notice that the electrons are in motion. We also 

 notice that the kinetic energy of two parallel electron 

 streams is greater, even though the velocity remains 

 the same, when the streams are closer together. The 

 inertia or mass, as we prefer to say, of an electron de- 

 pends, then, upon its velocity and upon the parallel 

 streams of electrons in its neighborhood. 



It has been found, however, that for velocities which 

 are small as compared to that of light the mass of the 

 electron is practically constant. If the mass at rest 

 is represented by m , while that for motion with a 

 velocity of v cm. /sec. is represented by m, and the 

 velocity of light by c cm. /sec. then 



v 2 /c 2 " 



gives values for m which have been checked by ex- 

 periment for speeds from about 0.3 to 0.8 that of light. 

 For experimental purposes electrons moving with these 

 high velocities are obtained from X-ray tubes or from 

 radioactive substances. 



For velocities below about 0.1 c. such as we meet in 



