g through any of the coils, is held midway
between the magnet cores by the two spiral springs S and T, which are under
slight but equal tension. The connections are as follows. The wires from
the winding on M are connected directly to the relay terminals F and H, as
are also the wires from one winding on N. The other winding on N is
connected in series with the battery C, ammeter B, and regulating
resistance R. {80}

[Illustration: FIG. 38.]

When the circuit of the battery C is completed, the coil of N, to which it
is connected, is energised, and the armature A is attracted against the
stop V. When in this position the tension of the spring S is released,
while the tension of the spring T is increased. As soon as the circuit of
the battery D is completed by means of the metal line print on the
transmitting machine, the current divides at the terminals F and H, a
portion flowing through the magnet coil M, and a portion through the
remaining winding on N. The current which flows through the winding on N
produces a magnetising effect equal to that caused by the other winding on
N, but since the two windings are of equal length and resistance, and since
the current flowing through the two windings is of equal strength but in
opposite directions, the result is to neutralise {81} the magnetising
effects produced by each winding, and consequently no magnetism is produced
in the cores.

The other portion of the current from D flows through the coil M, and it
becomes magnetised at the same time that the coil N becomes demagnetised.
The armature A is attracted by M against the stop X, and this attraction is
assisted by the spring T, which was under increased tension. The conditions
of the springs are now reversed, the spring S being under increased
tension, while the tension of the spring T is released.

As soon as the current from D is broken, the magnetism disappears from M,
the neutralising current in N ceases, and N once more becomes magnetised,
owing to the current which still flows through one winding from C; the
armature is therefore again attracted by N, assisted by the spring S. The
current flowing through the two windings of N must be perfectly equal, and
the regulating resistance R, and ammeters B and B', are inserted for
purposes of adjustment. The current from C must flow in a direction
opposite to that which flows from D.

[Illustration: FIG. 39.

H, H', containers; M, mercury; E, paraffin oil; T, T', termina