pring _4_. A pin _8_ carried by the crank shaft, rests in a
V-shaped notch in the end of the hub _6_ and as a result, when the
crank is turned the pin rides on the surface of this notch before the
large gear wheel starts to turn, and thus moves the crank shaft _5_ to
the right and breaks the contact between it and the spring _4_. Thus,
as long as the generator is being operated, its armature is connected
in the circuit of the line, but as soon as it becomes idle the
armature is automatically short-circuited. Such devices as this are
termed _automatic shunts_.

In still other cases it is desirable to have the generator circuit
normally open so that it will not affect in any way the electrical
characteristics of the line while the line is being used for talking.
In this case the arrangement is made so that the generator will
automatically be placed in proper circuit relation with the line when
it is operated.

[Illustration: Fig. 75. Generator Cut-in Switch]

A common arrangement for doing this is shown in Fig. 75, wherein the
spring _1_ normally rests against the contact pin of the armature and
forms one terminal of the armature circuit. The spring _2_ is adapted
to form the other terminal of the armature circuit but it is normally
insulated from everything. The circuit of the generator is, therefore,
open between the spring _2_ and the shaft _3_, but as soon as the
generator is operated the crank shaft is bodily moved to the left by
means of the =V=-shaped notch in the driving collar _4_ and is thus
made to engage the spring _2_. The circuit of the generator is then
completed from the spring _1_ through the armature pin to the armature
winding; thence to the frame of the machine and through shaft _3_ to
the spring _2_. Such devices as this are largely used in connection
with so-called "bridging" telephones in which the generators and bells
are adapted to be connected in multiple across the line.

A better arrangement for accomplishing the automatic switching on the
part of the generator is to make no use of the crank shaft as a part
of the conducting path as is the case in both Figs. 74 and 75, but to
make the crank shaft, by its longitudinal movement, impart the
necessary motion to a switch spring which, in turn, is made to engage
or disengage a corresponding contact spring. An arrangement of this
kind that is in common use is shown in Fig. 76. This needs no further
explanation than to say that the crank shaft is p