CHAPTER III.
INDUCED MAGNETISM.
=EXPERIMENT 24. To find whether we can magnetize a piece of iron without touching it with a magnet.=
_Apparatus._ Horseshoe magnet, H M; iron filings, I F (Fig. 11).
=52. Directions.= (A) Hold the armature of the magnet in a vertical position (Fig. 11), its lower end being directly in a little pile of iron filings.
(B) Bring the N pole of H M near the upper end of A, but do not let them touch each other.
(C) Keeping A and the pole of H M the same distance apart, lift them. Do any filings cling to A?
(D) Without moving or jarring A, take H M away from it and note result upon the filings.
_=53. Temporary Magnetism; Induced Magnetism.=_ The armature, A, was induced to become a magnet without even touching H M. Its magnetism was _temporary_, however, as the filings dropped as soon as the _inductive action_ of H M was removed. A small amount of residual magnetism (44) remained in A. Soft iron is exceedingly valuable, because it has very little retentivity (44), and because it can be easily _magnetized by induction_. The armature was made of soft iron. It had _induced magnetism_. It was a _temporary magnet_.
=EXPERIMENT 25. To find whether a piece of steel can be permanently magnetized by induction.=
_Apparatus._ An unmagnetized sewing-needle; horseshoe magnet, H M; iron filings; sheet of stiff paper.
=54. Directions.= (A) Test the needle for magnetism.
(B) Place the unmagnetized needle upon the paper, then move H M about immediately under it, so that the needle will be attracted.
(C) Test the needle again for permanent magnetism.
=EXPERIMENT 26. To study the inductive action of a magnet upon a piece of soft iron.=
_Apparatus._ Horseshoe magnet, H M; iron filings, I F; a piece of soft iron wire about an inch long, I W (Fig. 12), placed upon the N pole of H M; compass, O C (No. 18), (ยง 32).
=55 Directions.= (A) Test the lower end of I W for magnetism with I F.
(B) Leaving I W upon the N pole of H M, test the pole at the lower end of I W with O C, to determine whether it is N or S.
(C) Jar I W (Exp. 19), then place it upon the S pole of H M, and again test the polarity of the lower end.
_=56. Polarization; Pole Pieces.=_ The wire, I W (Fig. 12), was acted upon by induction (Exp. 24) and behaved like a magnet. Poles were produced in it, so we say that the wire was _polarized_. Pieces of iron, placed upon the poles of a magnet, are called _pole pieces_. It should be noted that the lower end of the wire has a pole _like_ the pole of H M, to which it is attached.
=EXPERIMENTS 27-30. To study pole pieces.=
_Apparatus for Experiments 27-30._ Horseshoe magnet, H M; soft iron wires; iron filings, I F.
=57. Directions.= (A) Suspend two wires, each about an inch long (Fig. 13) from one pole of H M. Do their lower ends attract or repel each other?
=EXPERIMENT 28.=
=58. Directions.= (A) Place the two wires just used so that one shall cling to the N pole of H M, and the other to the S pole of H M (Fig. 14).
(B) Bring the lower ends of the wires near each other. Do they attract or repel each other?
=EXPERIMENT 29.=
=59. Directions.= (A) Bend a 2-inch iron wire, as in Fig. 15, and place it upon the poles of H M.
(B) See if its central part, marked X, will strongly attract filings.
=EXPERIMENT 30.=
=60. Directions.= (A) Bend the wire just used a little more, and place its ends upon _one_ pole of H M (Fig. 16).
(B) See if the iron filings and small wires will cling to its central part.