tion of the keys at the distant
ends. Hence, two messages can be sent in opposite directions over the
same line simultaneously.

The reader will undoubtedly see quite clearly from the above system,
which rests upon varying the STRENGTH of the current, that two messages
could not be sent in the same direction over the one line at the same
time. To accomplish this object Edison introduced another and distinct
feature--namely, the using of the same current, but ALSO varying its
DIRECTION of flow; that is to say, alternately reversing the POLARITY
of the batteries as applied to the line and thus producing corresponding
changes in the polarity of another specially constructed type of relay,
called a polarized relay. To afford the reader a clear conception of
such a relay we would refer again to Fig. 1 and its explanation, from
which it appears that the polarity of a soft-iron bar is determined not
by the strength of the current flowing around it but by the direction
thereof.

With this idea clearly in mind, the theory of the polarized relay,
generally called "polar" relay, as presented in the diagram (Fig. 7),
will be readily understood.

A is a bar of soft iron, bent as shown, and wound around with insulated
copper wire, the ends of which are connected with a battery, B, thus
forming an electromagnet. An essential part of this relay consists of
a swinging PERMANENT magnet, C, whose polarity remains fixed, that end
between the terminals of the electromagnet being a north pole. Inasmuch
as unlike poles of magnets are attracted to each other and like poles
repelled, it follows that this north pole will be repelled by the north
pole of the electromagnet, but will swing over and be attracted by
its south pole. If the direction of flow of current be reversed, by
reversing the battery, the electromagnetic polarity also reverses and
the end of the permanent magnet swings over to the other side. This
is shown in the two figures of Fig. 7. This device being a relay, its
purpose is to repeat transmitted signals into a local circuit, as before
explained. For this purpose there are provided at D and E a contact and
a back stop, the former of which is opened and closed by the swinging
permanent magnet, thus opening and closing the local circuit.

Manifestly there must be provided some convenient way for rapidly
transposing the direction of the current flow if such a device as the
polar relay is to be used for the reception of telegra