the small
circular opening results in connecting the earth or a common return
conductor to one pin of each set of ten pins, the pin grounded in each
set depending upon the numerical value of the digit for which the dial
is set.

The circle of contact pins is set in an insulating disk, the signal
transmitting brush operates upon the pins on one side of the disk, and
electrical fingers attached to the dials operate upon the pins on the
other side of the disk. The escapement wheel is a single toothed disk
attached directly to the shaft which carries the signal brush and its
pallet is attached rigidly to the magnet armature.

[Illustration: Fig. 403. Desk Stand with Signal Transmitter Removed]

Once a call has been turned in, the entire subscriber's station
equipment is locked beyond power of the subscriber to tamper with it in
any way, rendering it impossible either to defeat the call which has
been started or to prevent the subscriber's station as a whole from
returning completely to normal position and thus restoring itself for
regular service. The key shown just below the signal transmitter in the
case of the desk stand, and at the right in the wall set, is for the
purpose of operating a relay at the central office which, in turn,
connects ringing current to the line of the subscriber with which
connection has been made, and thus actuates the call bell.

As the number set up at the signal transmitter remains in full view
until reset for some other number, it is easily checked by inspection
and also lessens the labor involved in making a second call for the same
line, which is frequently necessary when the line is found busy the
first time called.

=Central-Office Apparatus.= The subscriber's lines are divided into
groups of one hundred lines each at the central office, each group being
served by a single unit of central-office apparatus. In a
central-office unit there is "sectional apparatus" which appears but
once for the unit of one hundred lines; "divisional apparatus" which
appears a number of times for each unit, depending upon the traffic; and
"line apparatus" which appears one hundred times for each unit or once
for each line.

The sectional apparatus comprises devices whose duties are, first, to
detect a calling line, and second, to assign to the calling line a set
of idle divisional apparatus which serves to perform the necessary
switching functions and complete the connection.

The sets of divisional