_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. =EXPERIMENT 41.= =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? =EXPERIMENT 42.= =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. =EXPERIMENT 43. To show that lines of force are on all sides of a magnet.= _Apparatus._ Our compass, O C (No. 18); horseshoe magnet, H M; glass tumbler, G T; sheet of stiff paper; iron filings, I F. Arrange as in Fig. 21. H M may be supported in a vertical position by placing paper, or a handkerchief, under it. The poles should just touch the stiff paper placed over the tumbler. [Illustration: Fig. 21.] =79. Directions.= (A) Sprinkle iron filings upon the paper, and study the resulting magnetic figure. (B) Place O C upon the paper in different positions. Does the magnetic needle always come to rest about parallel to the lines of filings? _=80. Discussion.=_ The student should keep in mind the fact that the filings in the magnetic figure show the approximate extent and form of the magnetic field simply in one plane. If the paper were held in some other position near the magnet (in a tilted position, for example,) the lines of filings would not be the same as those produced in Exp. 40-42. The lines of force come out of every side of the N pole. When a magnetic needle is placed in any magnetic field, its N pole points in the direction in which the lines of force are passing; that is, it points towards the S pole of the magnet producing the field.