730 Prof. A. S. Eve on Changes in Velocity in an 
negative charges on the active plate, then the observed effect 
will not correspond with that calculated. 
In order to test this point experimentally, a capsule con- 
taining 1*5 mgs. of radium was placed so that the ft rays 
passed through a small hole in a block of wood, and entered 
normally the centre of the face of the electroscope. The 
diameter of the hole was 1*5 cm., and the length of it was 
10 cms. The block of wood was covered with thin aluminium 
foil, was insulated on a block of paraffin, and charged as 
previously described. The observed difference for positive 
and negative potentials was 6'3 per cent., or about two-thirds 
of the calculated value (inner curves, fig. 2). It must 
be borne in mind that the ft rays of radium always include 
secondary rays, and that a group of ft rays has a very wide 
range of velocity, from almost that of light to that of the 
slowest electrons produced by secondary, tertiary, or higher 
order radiation. The value for the velocity of projection of 
the ft particles assumed in the above calculation was selected 
from Allen's paper"*, as being that derived under conditions 
most similar to those obtaining in my work. 
In this case, as with the a particle, no allowance has been 
made for the absorption and scattering by air between the 
plate and the electroscope. Under more rigorous experimental 
conditions, it is possible that this method might throw light 
on the ionizations produced for various velocities of the 
electrons. 
Secondary Rays. 
The method described in this paper was next applied to 
the investigation of the nature of the secondary rays from 
various substances due to the ft and 7 rays from radium. 
About 14 mgs. of pure radium bromide were sealed in two 
thin glass test-tubes. The radiators were 5 cms. thick and 
measured 22 x 22 cms. (fig. 3). They were insulated on 
blocks of paraffin and charged by the Wimshurst machine. 
A line from the radium to the centre of the radiating plate 
made an angle of 60° with the normal. When the radiator 
is charged to a high potential the primary rays will be repelled 
from, or attracted to, the radiator to an extent depending on 
the sign and the magnitude of the potential. This variation 
of the primary rays might have been avoided by using 7 rays 
only ; but 7 rays are always accompanied by secondary rays 
from the screen which cuts off the ft rays. Moreover, if all 
the radiators are of the same size and shape, and if they are 
placed in turn in the very same position, then, for any given 
* Phys. Keyiew, Aug. 1906. 
