New Self-Registering Electrometer. 295 
described several forms of electrometer involving these prin- 
ciples in the proceedings of the Royal Society of London, 
“Nicol’s Encyclopedia’ (1860), and in several other 
works. 
Sir William Thomson has shown in several communica- 
tions to the Royal Society of London, thatif the sensitive or 
movable portions of any of the ordinary electrometers be kept 
charged witheither positive or negative electricity, it not only 
becomes highly sensitive to the slightest electric force, but at 
the same time becomes an electroscopic electrometer, indi- 
cating at once whether the impressions made on it are from 
positive or negative electricity ; and his instruments were so 
constructed that the sensitive movable part or indicator was 
in connection with the inner coating of a Leyden jar, charged 
with either positive or negative electricity, the charge being 
maintained in the jar over long periods without any great 
change by protecting the jar from external influences 
by a surrounding metal case, and further by artificially 
drying the air within the case. The sensitive parts were 
acted on by insulated pieces of metal, which he styled elec- 
trodes, placed in a proper position near to the sensitive parts. 
The electrode or electrodes were generally arranged so that 
they could either be placed alternately in connection with 
the earth or with the body to be tested, or one constantly in 
connection with the body to be tested, and the other with 
the earth. 
Now, supposing the inner coating of the jar to be charged 
positively, the sensitive parts are also similarly charged, and 
would be repelled by any other body semilarly and attracted 
by a body differently electrified. The surface of the earth is 
as a rule negative, and if the electrode were connected with 
the earth, the sensitive part would be attracted ; but if the 
electrode were attached to a positively electrified body, an 
atmospheric collector for instance, it would be repelled, and 
this attraction and repulsion would be a joint measure of 
the amount of electric force on the earth’s surface or in the 
_ atmospheric collector and the inside coating of the jar ; the 
latter being measurable, the amount of the other forces 
becomes also measurable. 
The mode of conveying to the electrode the electric poten- 
tial of the atmosphere devised by Sir William Thomson, is 
very simple and effective ; it consists of an insulated vessel 
of water from which a pipe terminating in a fine nozzle pro- 
jects into the air ; from this nozzle the water issues in a very 
