of each of the magnets shall freely enter the respective
bores of the two bobbins fixed upon the same plate, and, when the whole
system is in equilibrium and the bobbins free from current, the two
branches of each of the magnets shall nearly coincide with the axes of
such bores. The magnets are not plane, but are curved so as to form
portions of a vertical cylinder whose axis coincides with the direction
of the suspension wire, and to which the axes of the bobbins are tangent
at their center, approximately to the points where the poles of the
magnets are situated.

[Illustration: FIG. 1. GRAY'S GALVANOMETER.]

The needles have been given this form so that their extremities shall not
touch the sides of the bore during considerable deflections.

In the instrument which the inventors, Messrs. T. & A. Gray, used in
their experiments upon the resistance of glass, the needles were arranged
so that their poles of contrary name were opposite.

[Illustration: FIG. 2.]

The system of needles is suspended from the extremity of a screw, p,
which passes into a nut, n, movable between two stationary pieces. On
revolving the nut, we cause the screw to rise or lower, along with the
entire suspended part, without twisting the thread.

The four bobbins are grouped for tension, and have a total resistance of
30,220 ohms. They contain 16,000 feet of No. 50 copper wire, forming
62,939 revolutions, nearly equally divided between the four bobbins. When
a current is passing through the bobbins, the poles of one of the
horseshoe magnets are attracted toward the interior of the corresponding
bobbins, while those of the other are repelled toward the exterior by the
two other bobbins. We thus have a couple which tends to cause the system
to revolve around the suspension axis. A mirror, which is fixed upon a
vertical piece of aluminum, a, gives, in the usual manner, a reflected
image upon a scale, thus allowing the deflections to be read. A
compensating magnet, M, is supported by a vertical column fixed to the
case, above the needles. This magnet may be placed in the different
azimuths by means of a tangential screw, t. The extremities of the bobbin
wires are connected with three terminals, T, T', T squared, and the
instrument may, by a proper arrangement, became differential. These
terminals, as well as the communicating wires, are insulated with
ebonite.

Thus arranged, the instrument is capable of making a deflection of one
division of