214 
Mr Campbell, The /3 Rays from Potassium. 
potassium is 2'7 x 10 -13 amperes : hence a change of i of 1 °/ 0 in 
the spontaneous ionisation current could be detected, or, in terms 
of radioactive processes, an increase or decrease of one a particle 
emitted every 30 seconds. 
Results. 
§ 7. The experiment worked more easity than the most 
sanguine expectations had anticipated. 
The current due to the potassium rays coming through the 
grid in the absence of the field was 280 arbitrary units, or 6 '25 °/ 0 
of the whole ionisation current in the testing vessel. (It must be 
remembered that the plates of the grid cut off all rays which are 
not emitted at a small angle with the vertical.) Fifteen observa- 
tions were taken of the effect of a field corresponding to a p.d. of 
8000 volts : they all indicated a decrease of the leak due to the 
field : the change of deflection of the leaf in ten minutes varied 
from + 7 7 to + 0’4 — mean + 3'8, equivalent to 39 units of current. 
Therefore the decrease of the potassium rays caused by the field is 
39 
280 
= 14 7c 
Similar experiments were made with the potassium sulphate 
replaced by a small packet containing uranium oxide. Two such 
series were made. In the first the current due to the uranium 
rays was 1075, the decrease due to the field 62 : in the second the 
current due to the rays was 1275 and the decrease due to the 
field 90. Hence we have 
Uranium (1) decrease 
62 
1075 
Uranium (2) decrease 
90 
1275 
= 6% 
mean 6'5 °/ o . 
Since the decrease caused by the field will vary to some extent 
with the distribution of the uranium oxide (since on this distri- 
bution depends the proportion of the rays which are given out at 
any particular angle) the difference between the two results need 
cause no surprise. 
§ 8. It was important to perform “ blank experiments ” with 
no active material in the tray to make sure that the effect of the 
field really depended on the presence of charged rays and not on 
some accidental disturbance. Thirty such blank observations were 
taken : they were performed in exactly the same way as the rest, 
except that there was no active material in the tray. The de- 
flections varied from — 5T to + 5’0 — mean — 0 027, equivalent to 
0‘28 units of current, less than one per cent, of the change with 
