364 Prof. J. A. McClelland on the Comparison 
A need never change by more than one volt, so that there is 
no trouble in keeping a constant current to the plate A. The 
constancy of the current in the above arrangement only 
depends on the constancy of the radiation from the uranium ; 
and numbers will be given to show how very constant this 
radiation is. The potential-difference required to produce 
the maximum current to A will depend on the distance 
between A and B; but itis well to have 200 volts available. 
C represents one of the condensers being compared ; and 
FE is a quadrant-electrometer. D is an insulating block of 
paratin, containing two mercury cups, a and b, one of which 
is connected to earth. A connecting piece L is shown, 
joining the mercury cups, and by means of a string arrange- 
ment L can be lifted out of the mercury cups and lowered 
again as desired, from a distance, so as to avoid induction 
effects produced by movements of the observer. 
As soon as L is lifted out of the mercury cups, the plate A, 
the condenser C, and the electrometer begin to charge up; 
and the time is observed during which the spot of light moves 
over, say, 100 scale-divisions. An exactly similar experiment 
is done with a second condenser C’ in the place of C. If the 
intervals of time are respectively ¢t and ¢t’, we have 
C+co_t 
CAt- aes 
where cis the capacity of the electrometer, the condenser AB, 
and the connecting wires. The capacity ¢ can be determined 
in the terms of, say, C, by taking an observation with C 
joined up as shown, and then an observation with C discon- 
nected, so that only ¢ is charged ; or the capacity ¢ may be 
determined once for all by comparing it in this way with a 
known capacity. We thus get the ratio C/C’. 
Accuracy of the Method. 
The accuracy of the method obviously depends simply on 
the constancy of the ionization current, and on the accuracy 
with which the time-intervals are measured. Numbers are 
given below to show how very constant the ionization current is. 
In practice it is well to screen the space between the plates A 
and B from air-currents, as such currents, if strong, may 
blow away the ionized air, and diminish the ionization-current. 
As regards the radioactive substances used, uranium is pre- 
ferable to thorium or radium, as it gives off no emanation. 
If radium or thorium is used, it should be in a closed vessel 
to prevent the emanation from escaping, otherwise the 
ionization-current will not be steady. 
