his that the line
circuits actually have three wires but that each line employs three
conductors, the two wires of the line and the earth. The three-wire
system will be referred to and described in detail, and from it the
principles of the two-wire system will be readily understood.

[Illustration: Fig. 382. Automatic Wall Set]

[Illustration: Fig. 383. Automatic Desk Stand]

=Subscriber's Station Apparatus.= The detailed operation of the
three-wire system may be best understood by considering the subscriber's
station apparatus first. The general appearance of the wall set is shown
in Fig. 382, and of the desk set in Fig. 383. These instruments embody
the usual talking and call-receiving apparatus of a common-battery
telephone and in addition to this, the signal transmitter, which is the
thing especially to be considered now. The diagrammatic illustration of
the signal transmitter and of the relation that its parts bear to the
other elements of the telephone set is shown in Fig. 384. It has already
been stated that the subscriber manipulates the signal transmitter by
rotating the dial on the face of the instrument. A clearer idea of this
dial and of the finger stop for it may be obtained from Figs. 382 and
383.

[Illustration: Fig. 384. Circuits of Telephone Set]

_Operation._ To make a call for a given number the subscriber removes
his receiver from its hook, then places his forefinger in the hole
opposite the number corresponding to the first digit of the desired
number. By means of the grip thus secured, he rotates the dial until its
movement is stopped by the impact of the finger against the stop. The
dial is then released and in its return movement it sends the number of
impulses corresponding to the first digit in the called number. A
similar movement is made for each digit.

In Fig. 384 is given a phantom view of the dial, in order to show more
clearly the relation of the mechanical parts and contacts controlled by
it. For a correct idea of its mechanical action it must be understood
that the shaft _1_, the lever _2_, and the interrupter segment _3_ are
all rigidly fastened to the dial and move with it. A coiled spring
always tends to move the dial and these associated parts back to their
normal positions when released by the subscriber, and a centrifugal
governor, not shown, limits the speed of the return movement.

The subscriber's hook switch is mechanically interlocked with the dial
so as to prevent t