Continuing the circuit through the winding itself
Continuing the circuit through the winding itself, it passes to the
center pin projecting from the left-hand end of the armature shaft;
thence to the spring _4_ which rests against this pin; and thence to
the terminal wire _3_.
Normally, this path is shunted by what is practically a short circuit,
which may be traced from the terminal _2_ through the frame of the
generator to the crank shaft _5_; thence to the upper end of the
spring _4_ and out by the terminal wire _3_. This is the condition
which ordinarily exists and which results in the removal of the
resistance and the impedance on the armature winding from any circuit
in which the generator is placed, as long as the generator is not
operated.
An arrangement is provided, however, whereby the crank shaft _5_ will
be withdrawn automatically from engaging with the upper end of the
spring _4_, thus breaking the shunt around the armature circuit,
whenever the generator crank is turned. In order to accomplish this
the crank shaft _5_ is capable of partial rotation and of slight
longitudinal movement within the hub of the large gear wheel. A spring
7 usually presses the crank shaft toward the left and into engagement
with the spring _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.