it from your end. If the wire is wound in a clockwise
direction, the S. pole will be nearest you; if in an anti-clockwise
direction, the N. pole. In Fig. 55 the N. poles are at the right end of
the coils, the S. poles at the left end; so the N. pole of the needle is
attracted to the right, and the S. pole to the left. When the current is
reversed, as in Fig. 56, the needle moves over. If no current passes, it
remains vertical.

METHOD OF REVERSING THE CURRENT.

[Illustration: FIG. 57.--General arrangement of needle-instrument
circuit. The shaded plates on the left (B and R) are in contact.]

A simple method of changing the direction of the current in a
two-instrument circuit is shown diagrammatically in Fig. 57. The
_principle_ is used in the Wheatstone needle instrument. The battery
terminals at each station are attached to two brass plates, A B, A^1
B^1. Crossing these at right angles (under A A^1 and over B B^1)
are the flat brass springs, L R, L^1 R^1, having buttons at their
lower ends, and fixed at their upper ends to baseboards. When at rest
they all press upwards against the plates A and A^1 respectively. R
and L^1 are connected with the line circuit, in which are the coils of
dials 1 and 2, one at each station. L and R^1 are connected with the
earth-plates E E^1. An operator at station 1 depresses R so as to
touch B. Current now flows from the battery to B, thence through R to
the line circuit, round the coils of both dials through L^1 A^1 and
R to earth-plate E^1, through the earth to E, and then back to the
battery through L and A. The needles assume the position shown. To
reverse the current the operator allows R to rise into contact with A,
and depresses L to touch B. The course can be traced out easily.

In the Wheatstone "drop-handle" instrument (Fig. 54) the commutator may
be described as an insulated core on which are two short lengths of
brass tubing. One of these has rubbing against it a spring connected
with the + terminal of the battery; the other has similar communication
with the - terminal. Projecting from each tube is a spike, and rising
from the baseboard are four upright brass strips not quite touching the
commutator. Those on one side lead to the line circuit, those on the
other to the earth-plate. When the handle is turned one way, the spikes
touch the forward line strip and the rear earth strip, and _vice versa_
when moved in the opposite direction.

SOUNDING INSTRUMENTS.

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