
of Capacities in Electrical Work. 369 
The method involves the use of a quadrant-electrometer, 
which to some may appear an objection to the method. The 
writer’s experience, however, is that no sensitive scientific 
instrument gives less trouble in working than a quadrant- 
electrometer, when it has once been put in good order. 
When the capacities being compared are small, no great 
sensitiveness of the. electrometer will be required—say 60 
millimetre scale-divisions for one volt with scale one metre 
from electrometer. When large capacities are being com- 
pared greater sensitiveness will be necessary, unless a very 
large quantity of uranium is used ; but there is no trouble 
in having an instrument one hundred times as sensitive as 
above. 
Some Experiments with this Method. 
(a) We shall first give some numbers to show the con- 
stancy of the ionization-current in the above arrangement, 
and the accuracy with which the time required to charge any 
system through a given range of potential can be measured. 
The system charged consisted of the electrometer, a capacity 
marked ‘C01 microfarad, and the condenser formed of the 
plates between which the uranium is placed. 
The time taken for the spot of hght to move over fift 
seale-divisions was taken with a stop-watch reading to fifths 
of a second. A series of seven observations was made, giving 
the following numbers, no observation being rejected. 
Time taken to move over 50 scale-divisions :— 
99°2 seconds. 
99°5 
99-0 
99-4 
Une 
oo ee, 
993%) ve.s§ 
Mean... 99°23 seconds. 
The agreement between these numbers is no better than that 
usually observed in other experiments ; in fact, not as good 
as in many other cases. 
(4) We shall now give the numbers observed in a com- 
parison of a condenser with a standard condenser, marked 
‘001 microfarad. We shall denote the capacity of the con- 
denser to be measured by ©, and the capacity of the electro- 
meter and other parts of the system by ec. | 
The electrometer in this experiment gave a deflexion of 
Phil. Mag. 8. 6. Vol. 7. No. 40. April 1904. 2C 
9) 
99 
7? 
