ground the
release wire of the first selector trunk extending back to the line
switch. Referring to Fig. 389, it will be seen that this path leads
through the contacts _13_ and _14_ and the release magnet to the
battery. It is by this means that the line switch is released, the
release impulse being relayed back from the first selector.

=Second Selector Operation.= For the purpose of considering the action
of the second selector, we will go back to the point where the first
selector had connected with a second selector trunk and where its side
switch had moved into its third position. In this condition, it will be
remembered, the trunk line was cut through to a second selector trunk
and all first selector apparatus cleared from the talking circuit.

The second selector chosen is one corresponding to the thousands group
as determined by the first digit of the called subscriber's number. The
circuits of a second selector are shown in Fig. 394 and it must be borne
in mind that the mechanical arrangements for producing the vertical and
the rotary movement of the shaft and for operating the side switch are
practically the same as those of the first selector. As in the first
selector, the sequence of operation is controlled by the successive
positions of the side switch, the first position permitting the
selection of the hundreds corresponding to the vertical impulses, the
second position allowing the selector to search for an idle trunk in
that hundred, and the third position cutting the trunk through and
clearing the circuit of obstructing apparatus.

_First Position of Side Switch._ The first thing that happens when the
subscriber begins to move his dial in the transmission of the second
series of selecting impulses is the sending of a preliminary impulse
over the rotary side of the line. This, in the case of the second
selector, energizes the rotary relay which, in turn, energizes the
private magnet; but the private magnet in the case of the second
selector can do nothing toward the release of the side switch because
the projection _5'_, on the side switch arm _5_, meets a projection on
the rear of the selector shaft which thus prevents the movement of the
side switch arm _5_ until the selector shaft has been moved out of its
normal position.

Immediately after the establishment of the connection to the selector,
the second set of selecting impulses comes in over the vertical wire
from the subscriber's station. Thes