se impulses passing in one direction only and over
the metallic circuit. After the proper party has been reached, the
ringing current is put on in the reverse direction.

[Illustration: Fig. 201. Central-Office Impulse Transmitter]

The operator establishes the number of impulses to be sent by placing
the pointer opposite the number on the dial corresponding to the
station wanted. The ratchet wheel is stepped around automatically by
each impulse of current from an ordinary pole changer such as is
employed in ringing biased bells. When the required number of impulses
has been sent, a projection, carried on a group of springs, drops into
a notch on the drum of the selector shaft, which operation instantly
stops the selecting current impulses and at the same time throws on
the ringing current which consists of impulses in the reverse
direction. So rapidly does this device operate that it will readily
follow the impulses of an ordinary pole changer, even when this is
adjusted to its maximum rate of vibration.

[Illustration: VIEW OF A LARGE FOREIGN MULTIPLE SWITCHBOARD]

_Operation._ Space will not permit a full discussion of the details of
the central-office selective apparatus, but a general resume of the
operation of the system may now be given, with the aid of Fig. 202,
which shows a four-station line with the circuits of three of the
stations somewhat simplified. In this figure Station A, Station B,
and Station D are shown in their locked-out positions, A and B having
been passed by the selection and ringing of Station C, while Station D
is inoperative because it was not reached in the selection and the
line is still broken at Station C. Station C, therefore, has
possession of the line.

When the subscriber at Station C raised his receiver in order to call
central, a "flash" contact was made as the hook moved up, which
momentarily grounded the limb _L_ of the line. (See Fig. 200.) This
"flash" contact is produced by the arrangement of the hook which
assures that the lower contact shall, by virtue of its flexibility,
follow up the hook lever until the hook lever engages the upper
contact, after which the lower contact breaks. This results in the
momentary connection of both the upper and the lower contacts of the
hook with the lever, and, therefore, the momentary grounding of the
limb _L_ of the line. This limb always being continuous serves, when
this "flash" contact is made, to actuate the line signal at the
cen