728 Prof. A. S. Eve on Changes in Velocity in an 
A Kelvin dynamo-static machine, kindly lent by Professor 
Owens, enabled me to work at a higher potential than with 
the Wimshurst machine. But sparks passed from the Th G 
plate and perforated the thin face of the electroscope, and 
ions then rushed inside. The holes could be repaired with 
very thin aluminium leaf, bat observations were difficult to 
make. 
It may be here stated that it is important to have the mica 
windows of the electroscope as small as possible, otherwise 
the readings of the gold-leaf are not the same for positive 
and negative charges of the leaf, owing to induction effect, 
electric wind, &c. It is well to have a large well-earthed 
sheet of wide-meshed wire-netting just in front of the electro- 
scope. This screens off the induction effect, and it does not 
interfere with the success of the experiment. The motion of 
the gold-leaf was remarkably steady and unaffected by the 
high -potential in its neighbourhood. 
This experiment with the a rays shows in a simple manner 
the fact, proved by Rutherford, and later by Becquerel, that 
the a. particle carries a positive charge, and it indicates 
that the length of the range of these particles can be increased 
or decreased to a slight extent by an electric field with lines 
of force in the direction of motion. 
j3 Rays. 
Active matter containing radium, spread over a brass plate 
10 cms. in diameter, was covered by just sufficient aluminium 
to absorb the a rays, and to prevent the escape of emanation. 
This, made by Dr. Riimelin, serves as a convenient /3-ray 
standard. The plate was placed in front of the electroscope, 
about 20 cms. from it, and it was charged to +30,000 volts. 
The total change in the ionization amounted to about 12 per 
cent, of the value when the plate was uncharged. A con- 
siderable part of this change is due to the bending of oblique 
rays, in or out of the electroscope according to the sign of the 
charge ; secondary rays from brass must be present. Using 
the dynamo-static machine and obtaining a higher voltage, 
about +50,000, a difference of more than 15 per cent, was 
produced by a reversal of the electric field. As the potential 
is gradually increased from zero, the ionizations for positive 
and negative charges on the brass plate are not equidistant 
from the mean value (fig. 2). It appeared easier to retard 
and decrease the rays entering the electroscope than to 
accelerate and increase them. It is unfortunate that there is 
so little evidence as to the relationship between the magnitude 
of the velocity of electrons and the ionization produced by 
