267 
Art. XI.] Cole, Experience with Capillary Electrometer. 
no ordinary form of electrometer or galvanometer can do, as 
for example, give an instantaneous record photographically of 
the variations of E. M. F. or current through one or more 
cycles of an alternating-current dynamo. 
My own experience with the capillary electrometer began 
with the three forms described by Ostwald in the volume already 
cited. Figures i, 2 and 3 (Plate LXII) will serve to recall their 
construction, without detailed description. The instrument 
consists essentially of two small bodies of mercury separated 
from each other by sulphuric acid. In each figure : 
^ is a glass tube of perhaps . 5 cm. diameter, one end of 
which is reduced to capillary dimensions, and which contains 
mercury electrically connected to one terminal of the instrument. 
^ is a larger tube or bulb of perhaps i cm. diameter, con- 
taining the other body of mercury, attached to the second ter- 
minal of the instrument. Connection between b and c is made by 
ay the capillary within which mercury and acid come in 
contact. 
e is the anode terminal. 
I have made and used about twenty instruments of these 
three types. The first form is very convenient and sensitive 
enough for many laboratory experiments. The one I have used 
most gives about one scale division (about i mm.) for .01 volt 
potential difference, but by using a smaller capillary this type 
can be made three or four times as sensitive. 
The second form is altogether better than the first and is 
used in connection with a cheap reading microscope to magnify 
its small deflections. It is an excellent form — the best of the three 
Ostwald describes — for general laboratory use. The ones that I 
have used most give a deflection of 14 to 35 divisions on the 
rotating head of a certain micrometer microscope, i960 of 
whose divisions equal one millimeter. In this type it is better 
to have the terminal platinum wires sealed into the glass at b 
andc. (Fig. 2). 
The third form is decidedly more sensitive, but is far 
less convenient to use and gets out. of order more easily — so 
easily indeed that only skilled experimenters can use it with 
