Du Moncel also tried
the difference between round armatures and flat ones, and found that a
cylindrical armature was only attracted about half as strongly as a
prismatic armature having the same surface when at the same distance.
Let us examine this fact in the light of the magnetic circuit. The
poles are flat. You have at a certain distance away a round armature;
there is a certain distance between its nearest side and the polar
surfaces. If you have at the same distance away a flat armature having
the same surface, and, therefore, about the same tendency to leak, why
do you get a greater pull in this case than in that? I think it is
clear that if they are at the same distance away, giving the same
range of motion, there is a greater magnetic reluctance in the case of
the round armature, although there is the same periphery, because,
though the nearest part of the surface is at the prescribed distance,
the rest of the under surface is farther away; so that the gain found
in substituting an armature with a flat surface is a gain resulting
from the diminution in the resistance offered by the air gap.
[Footnote 1: "La Lumiere Electrique," vol. ii.]
POLE PIECES ON HORSESHOE MAGNETS.
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