Theories of Electricity 151 



speeds but will increase for velocities approaching that 

 of light. 



The question how far the mass of an electron results 

 from the electric charge associated with it can be put to 

 the test of experiment. For this purpose it is necessary 

 to determine the mass of the electron at different speeds, 

 and to compare the results with those to be expected 

 from theory. Radium is an ideal source of electrons 

 for such experiments, for it expels electrons over a wide 

 range of velocity and some of the swiftest have a velocity 

 equal to 95 per cent, of that of light. Kaufmann by an 

 ingenious method determined the mass of the electrons 

 projected from radium at different speeds and found, 

 as theory had anticipated, that the mass was not constant 

 but increased rapidly as the velocity of light was ap- 

 proached. By comparison of theory with experiment, 

 he found that the mass of an electron was purely elec- 

 trical in origin and that there was no necessity to suppose 

 that the charge was distributed on a material nucleus. 

 This was a most important and far-reaching conclusion. 

 The electron is not matter at all in the ordinary sense, 

 but a disembodied electrical charge, possessing, however, 

 the characteristic property of mass in virtue of its motion. 



But we have seen that the electron is a constituent of 

 all matter and for ordinary speeds has a mass of about 

 i/iooo of that of the hydrogen atom. The mass of the 

 hydrogen atom would thus be explained if it consisted 



