
of Capacities in Electrical Work. 363 
the ratio of two capacities by simply observing two intervals 
of time. 
The method will probably have occurred to any one who 
has been using radioactive substances ; but as many workers 
have occasion to compare capacities accurately who are not 
using radioactive substances, I have thought it advisable to 
make a few experiments showing the accuracy of the method, 
and showing also how small a capacity can be detected and 
measured by it. 
To use the method it is not necessary to have a supply of 
radium, as the title of the paper might suggest ; uranium is 
even better in some respects, and uranium is to ‘be found in 
every laboratory. 
Description of Apparatus and Method of Working. 
A and B are two insulated metal plates, one of which, B, can 
be joined to one terminal of a battery of small storage-cells, the 
other terminal of which is to earth. The battery n may ose 
of 100 or more small test-tube cells, so that B can be kept 
at 200 volts or higher. 

STORAGE 
ra CELLS 
B 
.2e_ 5» FARTH 
A few grammes of, say, uranium nitrate are spread ona 
sheet of paper, and placed on the plate A. The radiation 
from the uranium ionizes the air between A and B: andso A 
gradually rises in potential if insulated, supposing B to be 
positive. As is well known, the ionization current thus 
obtained between two plates increases at first as the potential- 
difference between the plates increases; but when this po- 
tential-difference is made sufficiently g oreat, the current attains 
amaximum, and does not further increase ie further increase 
of potential-difference between the plates. If then B is kept 
at a sufficiently high potential, small changes in this potential, 
due to the potential of the battery falling, will produce no 
effect ; and, again, in making an observation, the potential of 
