Hughes, F.R.S., which he has devised in the course of
carrying out his researches on the differences between different kinds
of iron and steel. The instrument is thus described in the _Proceedings
of the Royal Society_:
"It consists of a delicate silk-fiber-suspended magnetic needle, 5 cm.
in length, its pointer resting near an index having a single fine black
line or mark for its zero, the movement of the needle on the other
side of zero being limited to 5 mm. by means of two ivory stops or
projections.
[Illustration]
When the north end of the needle and its index zero are north, the
needle rests at its index zero, but the slightest external influence,
such as a piece of iron 1 mm. in diameter 10 cm. distant, deflects the
needle to the right or left according to the polarity of its magnetism,
and with a force proportional to its power. If we place on the opposite
side of the needle at the same distance a wire possessing similar
polarity and force, the two are equal, and the needle returns to zero;
and if we know the magnetic value required to produce a balance, we know
the value of both.
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