[KENNEDY] ON THE NUMBER OF DELTA PARTICLES EXPELLED 33 
of the radiations emitted. This was done with the air in 8 at a number 
of different pressures. A set of these results for 25-6 mm., -95 mm. 
and +001 mm pressure is given in Table I and curves which illustrate 
them are shewn in Fig. III. 
At the pressure 25-6 mm. and 5-6 mm. the charge acquired was 
negative and in the two cases limiting potentials of approximately-318 
and +202 volts respectively were reached. 
The explanation of this acquisition of a negative charge by the 
electrode N was taken to be the existence of a volta difference of poten- 
tial between the copper plate and the silver lining of the vessel S. At 
the lower pressures, however, as may be seen from the curves, the system 
acquired a positive charge which as Sir J. J. Thomson* has shewn is due 
to the delta rays carrying a negative charge from the polonium coated 
plate in excess of the positive charge carried from the same plate, by 
the a rays. 
To get an accurate measure of the charge thus carried off by the 
delta rays the pressure was reduced in the vessel 8 until the McLeod 
gauge attached to the pump indicated a pressure of -001 mm. of mercury. 
An observation was then made on the rate at which the plate N ac- 
quired a negative charge. By means of the trap R the vessel was then 
cut off from the pump, and the liquid air was placed round the vessel 
M. After it was certain that the charcoal had absorbed as much of 
the remaining air as possible a set of readings was again taken on the 
rate at which the plate N charged up. 
This rate, however, was found to be practically the same as that 
obtained just before the liquid air was applied, and from the similarity 
in these two sets of readings it was considered that any current in the 
gas at -001 mm. pressure due to ionization was negligible. 
The numbers given in Table I and the graph representing them 
may therefore be taken to represent a measure of the rate at which the 
plate N emitted particles carrying negative electricity in excess of those 
emitted which carried positive charges. 
Since the number of @ particles leaving N per second was deter- 
mined in the first part of the investigation it was a simple matter to 
calculate the rate at which the delta particles were emitted and so de- 
duce the number shot off per @ particle. The calculation is as follows :— 
Calculation I.—Determination of number of @ particles emitted 
per second. 
* J. J. Thomson, Proc. Can. Phil. Soc. 13, 49, 1905. 
