[KENNEDY] ON THE NUMBER OF DELTA PARTICLES EXPELLED 31 
quadrant electrometer and exposed in an ionizing chamber C. This 
vessel which was cylindrical in form was 15 cms. in length and 12 cms. 
in diameter. 
As the range of the @ particles in air at atmospheric pressure has 
been given by Geiger* as 3.86 cms. it is clear that this air chamber was 
of ample size to insure that all the a rays expelled by the polonium on 
N. were absorbed by the air in it before they could reach its walls. 
With this apparatus the ionization currents were measured with 
various voltages applied to the receiver and the results of these measure- 
ments are shewn by the curve in Fig. Il. From the numbers there 
plotted it will be seen that saturation was obtained with between 200 
and 250 volts applied to the receiver C. 
The electrical capacity of the system, which included a condenser 
in this part of the investigation, was then determined and from the 
known value of the saturation current it was easy to deduce, assuming 
the charge on an ion to be 4-65 x 107° E.S. U. the number of ions made 
in the gas per second by all the a particles emitted by the polonium. 
As Geiger* has shewn that the number of ions made by each a 
particle emitted by polonium in its passage through a column of air is 
approximately 1-62x10° it follows since the number of ions produced 
per second by all the a particles was determinate that the number of 
a particles expelled per second by the sample of polonium used could 
at once be calculated. 
When the number of @ paiticles emitted per second at any par- 
ticular time by a sample of polonium is ascertained, one is enabled by 
the known rate of decay of polonium to calculate the number of @ par- 
ticles emitted per second by the same sample of polonium at any later 
time. 
After the rate at which the deposit on the copper plate N emitted 
a particles had been determined this plate was placed in a second 
chamber S made of glass silvered on its inner surface. The rod which 
carried the plate N was supported in § by a plug of ebonite freshly 
cleaned. It was protected by a guard tube as shewn in the figure and 
was connected to the free quadrants of the electrometer. 
To the chamber $ there was attached a second glass tube partly 
filled with cocoanut charcoal. This chamber § was also connected to a 
mercury pump and in this connection a trap R was inserted which en- 
abled one to cut the pump off and push the evacuation to the lowest 
possible limit by cooling the charcoal chamber with liquid air. 
*Geiger. Proc. Roy. Soc. A 82, p. 486, July 31, 1909. 
*Geiger, loc. cit. 
