[Illustration:  Fig 74.  Generator Shunt Switch]

A simple and much-used arrangement for this purpose is shown in Fig. 74, where 1 is the armature; 2 is a wire leading from the frame of the generator and forming one terminal of the generator circuit; and 3 is a wire forming the other terminal of the generator circuit, this wire being attached to the spring 4, which rests against the center pin of the armature so as to make contact with the opposite end of the armature winding to that which is connected with the frame.  The circuit through the armature may be traced from the terminal wire 2 through the frame; thence through the bearings to the armature 1 and through the pin to the right-hand side of the armature winding.  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An arrangement is provided, however, whereby the crank shaft 5 will be withdrawn automatically from engaging with the upper end of the spring 4, thus breaking the shunt around the armature circuit, whenever the generator crank is turned.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.