In order to balance any wire or piece of iron placed
in a position east and west, a magnetic compensator is used, consisting
of a powerful bar magnet free to revolve upon a central pivot placed
at a distance of 30 or more cm., so as to be able to obtain delicate
observations. This turns upon an index, the degrees of which are
marked for equal degrees of magnetic action upon the needle. A coil of
insulated wire, through which a feeble electric current is passing,
magnetizes the piece of iron under observation, but, as the coil itself
would act upon the needle, this is balanced by an equal and opposing
coil on the opposite side, and we are thus enabled to observe the
magnetism due to the iron alone. A reversing key, resistance coils, and
a Daniell cell are required."
The general design of the instrument, as shown in a somewhat crude form
when first exhibited, is given in the figure, where A is the magnetizing
coil within which the sample of iron or steel wire to be tested is
placed, B the suspended needle, C the compensating coil, and M the
magnet used as a compensator, having a scale beneath it divided into
quarter degrees.
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