110 NATURE OF THE RADIATIONS [CH. 
When the whole apparatus was placed in a vacuum, a P.D. of 
from 2000 to 5000 volts could be applied between the plates 
without a spark. The rays were deflected in their passage through 
the electric field and produced what may be termed an electric 
spectrum on the plate. 
If a magnetic field is superimposed parallel to the electric field 
by means of an electromagnet, a magnetic spectrum is obtained 
perpendicular to the electric spectrum. The combination of the 
two spectra gives rise to a curved line on the plate. Disregarding 
some small corrections, it can readily be shown that if y and z are 
the electric and magnetic deviations respectively, 
= Ky ceccecceece-corvesesessssssees 3 Fi 
B ji (3) 
e B 
and Saar wx (aisle siheioroit Se acral ae eee (4). 
From these two equations, combined with (1) and (2), we 
obtain 
where «, «,, «, are constants. 
Equation (5) gives the curve that should be obtained on the 
plate according to the electromagnetic theory. This is compared 
by trial with the actual curve obtained on the plate. 
In this way Kaufmann! found that the value of e/m decreased 
with the speed, showing that, assuming the charge constant, the 
mass of the electron increased with the speed. 
The followmg numbers give some of the preliminary results 
obtained by this method. 

Velocity of electron | — 
| m 

2°36 x 10° cms. per sec. 1°31 x 107 
2°48 53 | Leos 
259 i | 0-97 x 107 
272) ie Poe e laOpriese Gs 
2°85 is i. 0:63 x 107 


1 Nachrichten d. Ges. d. Wiss. zu Gitt., Nov. 8, 1901. 
" 
