and their reduced ends passed through holes in flanges extending from
the pole pieces, and riveted. The bearing plates in which the armature
is journaled are then secured to the ends of these pole pieces, as
will be shown in subsequent illustrations. This assures proper
rigidity between the pole pieces and also between the pole pieces and
the armature bearings.

The reason why this degree of rigidity is required is that it is
necessary to work with very small air gaps between the armature core
and its pole pieces and unless these generators are mechanically well
made they are likely to alter their adjustment and thus allow the
armature faces to scrape or rub against the pole pieces. In Fig. 71
one of the permanent horseshoe magnets is shown, its ends resting in
grooves on the outer faces of the pole pieces and usually clamped
thereto by means of heavy iron machine screws.

With this structure in mind, the theory of the magneto generator
developed in connection with Fig. 68 may be carried a little further.
When the armature lies in the position shown at the left of Fig. 71,
so that the center position of the core is horizontal, a good path is
afforded for the lines of force passing from one pole to the other.
Practically all of these lines will pass through the iron of the core
rather than through the air, and, therefore, practically all of them
will pass through the convolutions of the armature winding.

When the armature has advanced, say 45 degrees, in its rotation in the
direction of the curved arrow, the lower right-hand portion of the
armature flange will still lie opposite the lower face of the
right-hand pole piece and the upper left-hand portion of the armature
flange will still lie opposite the upper face of the left-hand pole