by which the variation is made to take place. One revolution of the
loop, therefore, results in a complete cycle of alternating current
consisting of one positive followed by one negative impulse.

The diagram of Fig. 68 is merely intended to illustrate the principle
involved. In the practical construction of magneto generators more
than one bar magnet is used, and, in addition, the conductors in the
armature are so arranged as to include a great many loops of wire.
Furthermore, the conductors in the armature are wound around an iron
core so that the path through the armature loops or turns, may present
such low reluctance to the passage of lines of force as to greatly
increase the number of such lines and also to cause practically all of
them to go through the loops in the armature conductor.

Armature. The iron upon which the armature conductors are wound is
called the _core_. The core of an ordinary armature is shown in Fig.
69. This is usually made of soft gray cast iron, turned so as to form
bearing surfaces at _1_ and _2_, upon which the entire armature may
rotate, and also turned so that the surfaces _3_ will be truly
cylindrical with respect to the axis through the center of the shaft.
The armature conductors are put on by winding the space between the
two parallel faces _4_ as full of insulated wire as space will admit.
One end of the armature winding is soldered to the pin _5_ and,
therefore, makes contact with the frame of the generator, while the
other end of the winding is soldered to the pin _6_, which engages the
stud _7_, carried in an insulating bushing in a longitudinal hole in
the end of the armature shaft. It is thus seen that the frame of the
machine will form one terminal of the armature winding, while the
insulated stud _7_ will form the other terminal.