Apparatus for Exps. 40–42. Horseshoe magnet, H M; iron filings, I F; sheet of stiff paper.
75. Directions. (A) Place H M, with its armature removed, flat upon the table, and cover it with the paper; then make the magnetic figure. ([Exp. 32].)
(B) Compare the number of well-defined curves at the poles with the number at the equator.
76. Directions. (A) Make the magnetic figure of H M with its armature in place.
(B) Is the attraction for outside bodies increased or decreased by placing the armature on H M?
77. Directions. (A) Lay H M flat upon the table, and place one or two matches between its poles and the armature; cover with paper as before, and make the magnetic figure. Do lines of force still pass through the armature?
78. Discussion; Resistance to lines of Force. It is evident, from the last 3 experiments, that lines of force will pass through iron whenever possible, on their way from the N to the S pole of a magnet. When the armature of a horseshoe magnet is in place, most of the lines of magnetic induction crowd together and pass through it rather than push their way through the air. Air is not a good conductor of lines of force; and the magnet has to do work to overcome the resistance of the air, when the armature is removed, in order to complete the magnetic circuit. This work causes a magnet to become gradually weaker. The soft iron armature is an excellent conductor of lines of force; it completes the magnetic circuit so perfectly that very little work is left for the magnet to do.