726 Prof. A. S. Eve on Changes in Velocity in an 
could be modified by a spark-gap, was measured by a Kelvin 
electrostatic voltmeter. 
Fig-. 1. 
9 
8 
oi- rays , 
TH- a . 
7 
1 -_-° + 
J fi 
40 
/ON/ZATfON 
SO 60 70 
— D! VS. PER M/Af. I 
The range of the a particle from Th C was found to be 
8*5 cms. by Rutherford and Hahn; and the bend at 
8 cms., shown in (fig. 1) is due to the entry of the a rays into 
the electroscope at that range, for it is known that the 
aluminium face of the electroscope is equivalent to 0*5 cm. of 
air. The curve (fig. 1) is drawn with the ionization effects 
as abscissae and the ranges as ordinates, following the well- 
known method of Bragg. 
At 9 cm. no a rays enter the electroscope, and a negative 
charge given to the active plate accelerates the fi rays, and 
a positive charge retards them. Oblique rays are bent towards 
or from the electroscope according to the sign of the charge. 
The charge actually observed was 12 per cent, of the mean 
value, and the effect for negative was greater than the effect 
for the positive charge. 
At 7 cms. the a rays were accelerated by a positive charge 
and retarded by a negative charge. The difference observed 
was 3 to 3*5 per cent, for potential-differences of +30,000 
volts, and the effect for a positive charge was then greater. 
At 8 cms. no change could be observed because the accelera- 
tion of the a rays balanced the retardation of the ft rays, 
and conversely. Only the very ends of the ranges of the a 
rays were inside the electroscope at this distance. 
If the field were uniform the a particles, projected in all 
directions, would describe parabolas, bent towards the electro- 
scope for a positive charge, and away from the electroscope 
for a negative charge. For the /3 particles the reverse state- 
ment is true. The effect may be compared to the motion of 
