To show in another way that few lines of force pass into the air, the student may lay the above combination upon the table and make a magnetic figure. (See Apparatus Book, p. 38, for method of making home-made compound magnets.)
In the case where a ring was placed between the poles of two bar magnets (Exp. 34), the lines of force from the N pole jumped across the first air-space. They then disappeared in the body of the ring, until they were obliged to jump across the second air-space, to get to the S pole. The weakness of the field in the central space was clearly shown by the filings. There were no stray lines of force passing through the air, because it was easier for them to go through the iron ring. This will be discussed again under "Dynamos and Motors." (See also § 78.)
=EXPERIMENTS 40-42. To study the magnetic field of the horseshoe magnet.=
_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.