rovided 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.

Such an arrangement is indicated diagrammatically in Fig. 77. Instead
of having one terminal of the armature winding brought out through the
frame of the generator as is ordinarily done, both terminals are
brought out to a commuting device carried on the end of the armature
shaft. Thus, one end of the loop representing the armature winding is
shown connected directly to the armature pin _1_, against which bears
a spring _2_, in the usual manner. The other end of the armature
winding is carried directly to a disk _3_, mounted _on_ but insulated
_from_ the shaft and revolving therewith. One-half of the
circumferential surface of this disk is of insulating material _4_ and
a spring _5_ rests against this disk and bears alternately upon the
conducting portion _3_ or the insulating portion _4_, according to the
position of the armature in its revolution. It is obvious that when
the generator armature is in the position shown the circuit through it
is from the spring _2_ to the pin _1_; thence to one terminal of the
armature loop; thence through the loop and back to the disk _3_ and
out by the spring _5_. If, however, the armature were turned slightly,
the spring _5_ would rest on the insulating portion _4_ and the
circuit would be broken.

[Illustration: Fig. 77. Pulsating-Current Commutator]

[Illustration: Fig. 78. Generator Symbols]

It is obvious that if the brush _5_ is so disposed as to make contact
w