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.

Another of Du Moncel's researches[2] relates to the effect of polar
projections or shoes--movable pole pieces, if you like--upon a
horseshoe electromagnet. The core of this magnet was of round iron 4
centimeters in diameter, and the parallel limbs were 10 centimeters
long and 6 centimeters apart. The shoes consisted of two flat pieces
of iron slotted out at one end, so that they could be slid along over
the poles and brought nearer together. The attraction exerted on a
flat armature across air gaps 2 millimeters thick was measured by
counterpoising. Exciting this electromagnet with a certain battery, it
was found that the attraction was greatest when the shoes were pushed
to about 15 millimeters, or about one-quarter of the interpolar
distance, apart. The numbers were as follows:

Distance between
shoes. Attraction,
Millimeters. in grammes.

2 900
10 1,012
15 1,025
25 965
40 890
60 550

[Footnote 2: "La Lumiere Electrique," vol. iv., p. 129.]

With a stronger battery the magnet without shoes had an attraction of
885 grammes, but with the shoes 15 millimeters apart, 1,195 grammes.
When one pole only was employed, the attraction, which was 88 grammes
without a shoe, was _diminished_ by adding a shoe to 39 grammes!

CONTRAST BETWEEN ELECTROMAGNETS AND PERMANENT MAGNETS.

Now I want particularly to ask you to guard against the idea that all
these results obtained from electromagnets are equally applicable to
permanent magnets of steel; they are not, for this simple reason. With
an electromagnet, when you put the armature near, and make the
magnetic circuit better, you not only get more magnetic lines going
through that armature, but