370 ~~ Comparison of Capacities in Electrical Work: 
Discussion of the Advantages of the Method. 
It is not necessary to compare this method in detail with 
the many other methods used in comparing capacities ; it will 
be sufficient to point out a few leading facts. 
Capacities may, of course, be compared by the electrometer 
without any use of radioactive substances by charging the 
unknown capacity to a potential which is measured by the 
electrometer, and then sharing the charge with a known 
capacity, and again measuring the potential. The method of 
working is not, however, as accurate as that described above, 
especially when the capacities are small. 
Capacities are often compared by charging them to the 
same potential, and discharging them through a_ ballistic 
galvanometer. The galvanometer-deflexion must be accu- 
rately read, and a. correction applied for damping, observations 
which cannot be made with the accuracy with which we can 
compare two intervals of time. Again, when the capacities 
are small, they must be charged or discharged through the 
galvanometer a great number of times per second, which 
requires carefully constructed apparatus to enable the number 
of charges to be accurately known, In addition it is somewhat 
difficult to be certain that the apparatus is working properly; 
for example, an error might arise through faulty insulation, 
and escape detection. 
The method of De Sauty is free from many of the objections 
mentioned above ; but others might be urged against it, and 
especially that it can be of little use when the capacities are 
very small. 
One of the chief advantages of the method described in this 
paper is that, from the nature of the apparatus used, it is 
scarcely possible for any serious source of error to come in 
without detection ; a faulty insulation, for example, can easily 
be guarded against. The only quantity requiring to be 
measured is an interval of time, which can be done with great 
accuracy. The ionization produced by the uranium keeps 
very constant throughout the time required to make a deter- 
mination, and there is no other quantity that requires to be 
kept very constant. The potential of the battery joined to 
one of the plates between which the uranium is placed may 
vary considerably between the observations and produce no 
effect, provided the potential is sufficiently great. 
The only objection that seems likely to be made to the 
method is the fact that it employs a quadrant-electrometer, 
the use of which in ordinary laboratory work has hitherto 
been discouraged. As stated above, the writer sees no reason 
for the reluctance to use electrometers when their usé can be 
