[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 provided on its end with an insulating stud 1, against which a switching spring 2 bears.  This spring normally rests against another switch spring 3, but when the generator crank shaft moves to the right upon the turning of the crank, the spring 2 disengages spring 3 and engages spring 4, thus completing the circuit of the generator armature.  It is seen that this operation accomplishes the breaking of one circuit and the making of another, a function that will be referred to later on in this work.

[Illustration:  Fig. 76.  Generator Cut-in Switch]

Pulsating Current.  Sometimes it is desirable to have a generator capable of developing a pulsating current instead of an alternating current; that is, a current which will consist of impulses all in one direction rather than of impulses alternating in direction.  It is obvious that this may be accomplished if the circuit of the generator be broken during each half revolution so that its circuit is completed only when current is being generated in one direction.