e projection on the spring _15_ in the opposite way and
the passing of each cam forces this vertical spring into engagement with
the ground spring. It will readily be seen, therefore, by a
consideration of the spacing of these cams on the segment and the finger
holes in the dial that the number of cams which pass the vertical spring
_15_ will correspond to the number on the hole used by the subscriber in
moving the dial.

Near the upper right-hand corner of the segment _3_, as shown in Fig.
384, there is another projection or cam _17_, the function of which is
to engage the rotary spring _13_ and press it into contact with the
ground spring. Thus, the first thing that happens in the movement of the
dial is for the projection _17_ to ride over the hump on the rotary
spring and press the contact once into engagement with the ground
spring; and likewise, the last thing that happens on the return movement
of the dial is for the rotary spring to be connected once to the ground
spring after the last vertical impulse has been sent.

If both the rotary and vertical sides of the line are connected with the
live side of the central-office battery, it follows that every contact
between the vertical and the ground spring or between the rotary and the
ground spring will allow an impulse of current to flow over the vertical
or the rotary side of the line.

We may summarize the action of these impulse springs by saying that
whenever the dial is moved from its normal position, there is, at the
beginning of this movement, a single rotary impulse over the rotary side
of the line; and that while the dial returns, there is a series of
vertical impulses over the vertical side of the line; and just before
the dial reaches its normal position, after the sending of the last
vertical impulse, there is another impulse over the rotary side of the
line.

The mechanical arrangements of the interrupter segment _3_ and its
associated parts have been greatly distorted in Fig. 384 in order to
make clear their mode of operation. This drawing has been worked out
with great care, with this in mind, at a sacrifice of accuracy in regard
to the actual structural details.

Ringing Springs:--The fourth group of springs in the subscriber's
telephone is the ringing group and embraces the springs _18_, _19_, and
_20_. The springs _19_ and _20_ are normally closed and maintain the
continuity of the talking circuit. When, however, the button attached to
the s