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
with the disk _3_ only during that portion of the revolution while
positive current is being generated, the generator will produce
positive pulsations of current, all the negative ones being cut out.
If, on the other hand, the spring _5_ may be made to bear on the