E will be out of contact with the magnet cores (A, A). A
spiral spring (G) secured to the rear ends of the arm and to the base,
respectively, serves to keep the rear end of the key normally in contact
with the screw F.

CONNECTING UP THE KEY AND SOUNDER.--Having made these two instruments,
we must next connect them up in the circuit, or circuits, formed for
them, as there must be a battery, a key, and a sounder at each end of
the line.

In Fig. 69 you will note two groups of those instruments. Now observe
how the wires connect them together. There are two line wires, one (A)
which connects up the two batteries, the wire being attached so that
one end connects with the positive terminal of the battery, and the
other end with the negative terminal.

[Illustration: _Fig. 69._ A TELEGRAPH CIRCUIT]

The other line wire (B), between the two stations, has its opposite ends
connected with the terminals of the electro-magnet C of the sounders.
The other terminals of each electro-magnet are connected up with one
terminal of each key by a wire (D), and to complete the circuit at each
station, the other terminal of the key has a wire (E) to its own
battery.

TWO STATIONS IN CIRCUIT.--The illustration shows station 2 telegraphing
to station 1. This is indicated by the fact that the switch F' of that
instrument is open, and the switch F of station 1 closed. When,
therefore, the key of station 2 is depressed, a complete circuit is
formed which transmits the current through wire E' and battery, through
line A, then through the battery of station 1, through wire E to the
key, and from the key, through wire D, to the sounder, and finally from
the sounder over line wire B back to the sounder of station 2,
completing the circuit at the key through wire D'.

When the operator at station 2 closes the switch F', and the operator at
station 1 opens the switch F, the reverse operation takes place. In both
cases, however, the sounder is in at both ends of the line, and only the
circuit through the key is cut out by the switch F, or F'.

THE DOUBLE CLICK.--The importance of the double click of the sounder
will be understood when it is realized that the receiving operator must
have some means of determining if the sounder has transmitted a dot or a
dash. Whether he depresses the key for a dot or a dash, there must be
one click when the key is pressed down on the screw head G (Fig. 62),
and also another click, of a different kind, when the