642 Mr. R. D. Kleeman on Different Kinds of 7 Rays 
rays could be wholly or partially compensated by a leak in 
the opposite direction in the uranium chamber, and the 
difference in the times of leak thereby rendered as large as 
convenient. 
The compensation was, however, rendered only partial, 
this affording a method o£ eliminating the loss of leak due 
to leakage over the insulation, and leakage in the screening 
tubing due to imperfect shielding from the 7 rays. Thus, if 
the times required to obtain a given deflexion with the leaks 
N and (N + M) be measured, the leaks for a given time (not 
corrected for loss of leak) can be calculated. Let these 
calculated leaks be denoted by N x and (N x + M x ) . Then the 
true value of M unaffected by leakage is given by sub- 
tracting 'N 1 from (JSTx + Mi), as will now be shown. When 
the system is charged, the loss of charge at any instant is 
approximately proportional to the deflexion of the electro- 
meter-needle. Therefore the loss of leak for a time dt is 
DA . dt, where D is the deflexion of the electrometer-needle 
and A is a constant. If n denote the leak per second if 
there were no loss of leak, then, since the loss of leak is 
small in comparison with the leak, D = £n approximately. 
If the leak be taken for a time t u the loss of leak is approxi- 
mately equal to f l tnA . dt, that is to t^An/2. If t^ denote 
the time required to obtain some given deflexion D b ti = T> 1 jn 
approximately. The loss of leak for the time t x is therefore 
~D 1 2 A/2n. Therefore, when the leak is calculated for a given 
time £ 2 > the value obtained is less than the correct value, the 
D 2 A t 
difference being equal to —^ — - , that is AD L t 2 /2. Since 
& n In ti ' 
D 1 and t 2 are taken the same in all the measurements, and A 
is a constant, the difference between the correct leak and the 
calculated leak is the same in each case. Consequently, 
when the calculated leak obtained when no radiator is used 
is subtracted from the calculated leak obtained when a 
radiator is used, the correct value of M unaffected by leakage 
is obtained. This method of eliminating the loss of leak was 
especially advantageous in these experiments since the loss 
of leak could not have been easily obtained directly, and it 
could not be neglected since its magnitude was not negligible 
in comparison with that of the leak. 
A set of measurements will now be described. A screen 
of some substance was placed in position, and the secondary 
radiation from a set of substances in terms of that from a 
standard substance determined. The screen was then re- 
placed by one of different material, and the whole process 
