Electric Field of the a, ft, and Secondary Rays. 727 
water issuing from a rose at the end of a hose directed to or 
from the earth. 
It is easy to estimate the change of velocity for an a, particle 
moving by any path from the plate at 30,000 volts to the 
electroscope at zero potential, for the change of kinetic energy 
is equal to the work done, so that 
^mv 2 — ^mu 2 = ±eY, 
or u 2 = u 2 + 2-Y. 
— m 
Here u is the velocity of projection, v of arrival, V is the 
potential-difference, e the ionic charge, and m the mass, 
expressed in E.M. units, and grams. The sign selected 
depends upon that of the charge. It has been proved 
by Rutherford and Hahn that for Th C, 
u=l-98xl0», - = o-6 xlO 3 , andV = 3xl0 12 . 
in 
Hence 
r 2 = (393 + 3'36)10 16 and »=19'91 x 10 8 , or ]9'74xl0 8 ; 
so that the total calculated difference per cent, for a change 
of sign of the potential is 0"8. 
The effect of absorption by the air has not been considered. 
The change in ionization observed is not necessarily pro- 
portional to the change of velocity produced. The large 
change observed at 7 cms. (3 per cent.) was due in part to 
the bending of the rays, and to the consequent increased 
length of path, of the oblique rays, which is in the electro- 
scope. Moreover, oblique rays which barely reach the 
electroscope when the plate is uncharged would be bent inside 
when the charge is given. The fact that no effect was observed 
at 8 cms. for a reversal of potential shows that the a ray 
change then balanced the ft ray change, and that the electric 
field altered the velocity of the normal a rays, and that the 
actual range of the a particles was increased or decreased 
according to the direction of the lines of force. I have 
endeavoured, hitherto without success, to observe this change 
of range with a Crookes spinthariscope, using an ebonite 
cylinder ; the eyepiece was earthed, and a wire, covered with 
radium C, was placed inside at the bottom on a plate raised to 
+ 30,000 volts. It is easy to show that the expected change 
of range must be small and difficult to detect, about 1*4 mm. 
for a reversal of potential*. 
* Rutherford, Phil. Mag. Oct. 1906. 
3C2 
