ISO PHYSICAL SCIENCE 



penditure of electro-magnetic energy, and thus the 

 electrified bodypossesseselectric inertia in addition 

 to its ordinary dynamical inertia. As long as the 

 velocity is small, this electric inertia is constant, 

 but an electrified body moving rapidly can be 

 shown mathematically to behave as though its 

 inertia, that is, its mass, were increased ; and, as 

 the velocity of light is approached, this apparent 

 electric mass grows very rapidly. Now some 

 experiments by Kaufmann, in which the masses 

 of the negative corpuscles emitted by radium were 

 investigated, are of intense interest in this con- 

 nection. The radium electrons move much more 

 rapidly than those found in cathode rays, though 

 in other respects electrons from the two sources 

 appear to be identical. With radium the veloci- 

 ties are so great that they approach closely that of 

 light. A speed of 2.85 x 10^^ centimetres a second 

 has been observed, that of light itself being 

 3.0 xio^^ At these enormous velocities, Kauf- 

 mann found that the value of efm, determined 

 from the magnetic and electric deflections, was con- 

 siderably diminished, a value of about one-third 

 the normal being obtained. Assuming that the 

 charge be constant, this means a threefold in- 

 crease in My the effective mass of the corpuscles. 



From the theory of electrons it is possible to 

 calculate what the increase of apparent mass 

 should be, on the assumption that the whole of 

 the mass of the corpuscle is an electrical manifesta- 

 tion, and, as we shall see in a future chapter, the 

 results of these and later experiments agree with 

 the calculated numbers. Such results are of 

 fundamental importance, both physically and 

 philosophically. It seems that the whole of the 



