of the line to the other so as to isolate the limbs from
each other while the signal transmitter is sending its impulses to the
central office.

Impulse Springs:--The third group embraces springs _13_, _14_, and _15_
and these are the ones by which the central-office switches are
controlled in building up a connection.

Something of the prevailing nomenclature which has grown up about the
automatic system may be introduced at this point. The movements of the
selecting switches at the central office are referred to as _vertical_
and _rotary_ for obvious reasons. On account of this the magnet which
causes the vertical movement is referred to as the _vertical magnet_ and
that which accomplishes the _rotary_ movement as the _rotary magnet_. It
happens that in all cases the selecting impulses sent by the
subscriber's station, corresponding respectively to the number of digits
in the called subscriber's number, are sent over one side of the line
and in nearly all cases these selecting impulses actuate the vertical
movements of the selecting switches. For this reason the particular limb
of the line over which the selecting impulses are sent is called the
_vertical limb_. The other limb of the line is the one over which the
single impulse is sent after each group of selecting impulses, and it is
this impulse in every case which causes the selector switch to start
rotating in its hunt for an idle trunk. This side of the line is,
therefore, called _rotary_. For the same reasons the impulses over the
vertical side of the line are called _vertical impulses_ and those over
the rotary side, _rotary impulses_. The naming of the limbs of the line
and of the current impulses _vertical_ and _rotary_ may appear odd but
it is, to say the least, convenient and expressive.

Coming back to the functions of the third group of springs, _13_, _14_,
and _15_, _15_ may be called the _vertical spring_ since it sends
vertical impulses; _13_, the _rotary spring_ since it sends rotary
impulses; and _14_, the _ground spring_ since, when the hook is up, it
is connected with the ground.

On the segment _3_ there are ten projections or cams _16_ which, when
the dial is moved, engage a projection of the spring _15_. When the dial
is being pulled by the subscriber's finger, these cams engage the spring
_15_ in such a way as to move it away from the ground spring and no
electrical contact is made. On the return of the dial, however, these
cams engage th