Iv] NATURE OF THE RADIATIONS 109 
Heaviside!, and Searle? that, according to the electromagnetic 
theory, a charge of electricity im motion behaves as if it had 
apparent mass. For small speeds this additional electrical mass 
is equal to a0 
3a 
rapidly as the speed of light is approached. It is very im- 
portant to settle whether the mass of the electron is due partly 
to mechanical and partly to electrical mass, or whether it can be 
explained by virtue of electricity in motion independent of the 
usual conception of mass. 
Slightly different formulae expressing the variation of mass 
with speed have been developed by J. J. Thomson, Heaviside, 
and Searle. To interpret his results Kaufmann used a formula 
developed by M. Abraham’. 
Let m,= mass of electron for slow speeds ; 
where a is the radius of the body, but it increases 
m = apparent mass of electron at any speed ; 
wu = velocity of electron ; 
V = velocity of light. 

Let 8 = 7 
Then it can be shown that 
m 
ae) AERC ee eae he coh ch et (il), 
~ isi ae e 
where an(3) = B L 38 log Tae s 1 slot oleracea (2). 
The experimental method employed to determine e/m and w is 
similar to the method of crossed spectra. Some strongly active 
radium was placed at the bottom of a brass box. The rays from 
this passed between two brass plates insulated and about 1:2 mm. 
apart. These rays fell on a platinum diaphragm, in which was 
a small tube about 0:°2 mm. in diameter, which allowed a narrow 
bundle of rays to pass. The rays fell on a photographic plate 
enveloped in a thin layer of aluminium. 
In the experiments the diaphragm was about 2 cms. from the 
active material and the same distance from the photographic plate. 
1 Collected Papers, Vol. 2, p. 514. 2 Phil. Mag. October, 1897. 
3 Phys. Zeit. 4, No. 1 b, p. 57, 1902. 
