ments are placed at each end of the line, as
in Fig. 6, we have a differential duplex arrangement by means of which
two operators may actuate relays at the ends distant from them, without
causing the operation of the relays at their home ends. In practice
this is done by means of a special instrument known as a continuity
preserving transmitter, or, usually, as a transmitter. This consists
of an electromagnet, T, operated by a key, K, and separate battery. The
armature lever, L, is long, pivoted in the centre, and is bent over
at the end. At a point a little beyond its centre is a small piece of
insulating material to which is screwed a strip of spring metal, S.
Conveniently placed with reference to the end of the lever is a bent
metallic piece, P, having a contact screw in its upper horizontal arm,
and attached to the lower end of this bent piece is a post, or standard,
to which the main battery is electrically connected. The relay coils
are connected by wire to the spring piece, S, and the armature lever is
connected to earth. If the key is depressed, the armature is attracted
and its bent end is moved upward, depressing the spring which makes
contact with the upper screw, which places the battery to the line, and
simultaneously breaks the ground connection between the spring and
the upturned end of the lever, as shown at the left. When the key is
released the battery is again connected to earth. The compensating
resistances and condensers necessary for a duplex arrangement are shown
in the diagram.

In Fig. 6 one transmitter is shown as closed, at A, while the other one
is open. From our previous illustrations and explanations it will be
readily seen that, with the transmitter closed at station A, current
flows via post P, through S, and to both relay coils at A, thence over
the main line to main-line coil at B, and down to earth through S and
the armature lever with its grounded wire. The relay at A would be
unresponsive, but the core of the relay at B would be magnetized and its
armature respond to signals from A. In like manner, if the transmitter
at B be closed, current would flow through similar parts and thus
cause the relay at A to respond. If both transmitters be closed
simultaneously, both batteries will be placed to the line, which would
practically result in doubling the current in each of the main-line
coils, in consequence of which both relays are energized and their
armatures attracted through the opera