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!