Electric Field of tlie a, /3, and Secondary Rays. 729 
them. According to Townsend the ionization decreases as 
the Telocity increases. Again, in the case of high velocity 
Fig. 2. 
POTENTIAL IN VOLTS 
electrons it is difficult to increase their speeds because, as 
Kaufmann and others have shown, there is an increase o£ 
mass as the velocity approaches that of light. There are then 
two reasons why the outer curves, shown in fig. 2, should 
not be symmetrical about the axis o£ x. 
It is easy to calculate the velocity of the /3 particle on its 
arrival at the electroscope. As before, we write 
But here we may take 
-=10 7 
m 
V. 
and u = 2-6xl0 10 . 
Hence, 
so that 
and 
u 2 = frl x 10 20 and 2- V=6 x 10 19 , 
m 
l ,2 = (67±6)10 19 
= 6-1, or 7-3, xlO 20 , 
v =2-46, or 2-70, xlO 10 . 
There is then a calculated difference equal to 9*2 per cent. 
of the mean value. But the difference produced in the- 
velocity need not cause a proportional difference of ionization 
in the electroscope. The result calculated above is inde- 
pendent of the path of the electron. But if the number of 
electrons entering the electroscope varies for positive and 
