of Radium, and Secondary Rays which they produce. 641 
held to one of the sides of each slit by means of a wedge* 
Therefore, when one of them was removed and again replaced, 
it always occupied its previous position. 
& — & 
Fi 9 .2. 
In order to obtain as large a leak as possible, the radiator 
B and the ionization-chamber D were made as large as was 
possible without causing any inconvenience. The leak in D, 
it will be observed, is approximately proportional to the 
product of the capacity of the ionization-chamber and the 
area of one side of the radiator, if the thickness of the 
radiator is kept constant. The length and breadth of each of 
the plates actually used as radiators were 28 and 2-4 cm. 
respectively. The ionization-chamber was of a cylindrical 
shape, its diameter was 18 cm., and its height approximately 
20 cm. 
The leak in the chamber D, as has already been pointed 
out, is partly due to the direct action of the <y rays pene- 
trating the lead block E. If we denote by N the leak per 
second due to these primary rays, and by M the leak due to 
the secondary rays, then the leak when the radiator is in 
position is given by (N-f-M). By observing the leak with 
no radiator we obtain the value of X. The value of M is 
then obtained by subtracting X from (N + M). 
In practice, the leak X or (X + M) was obtained by ob- 
serving the time taken by the electrometer-needle to reach 
a given deflexion. The deflexion for a given time could 
then be easily calculated. In this manner, the error due to 
the change of capacity of the electrometer with deflexion of 
needle was obviated. It will be observed, however, that if 
X is much greater than M (in these experiments this was 
the case), the times required to obtain a given deflexion 
when observing the leaks X and (X + M) would differ from 
one another by much less than the time of either leak. The 
possible error that is attached to the value of this difference 
is therefore large; and the error that is attached to the value 
of M, which depends on this difference, is therefore also 
large. This defect in the method was obviated by the use 
of the uranium chamber. The leak due to the primary 
