(B) Break the needle at its center, and test, with O C, the two new ends produced at that point. Remember that repulsion is the test for polarity.
39. Discussion; Consequent Poles. Iron filings cling to a magnet where poles are located. In this case, two small magnets were made in one piece of steel; they had a common S pole at the center. The pointing power ([§ 25]) of such a magnet is very slight; would it have any pointing power if we could make the end poles of equal strength? Intermediate poles, like those in the needle just discussed, are called consequent poles. Practical uses are made of consequent poles in the construction of motors and dynamos.
EXPERIMENT 17. To study consequent poles.
Apparatus. An unmagnetized sewing-needle; horseshoe magnet, H M (No. 16); iron filings (No. 17); compass (No. 18).
40. Directions. (A) Let w, x, y, and z stand for four places along the body of the needle, w being at its point and z at its head.
(B) Touch w with the N pole of H M, x with the S pole, y with the N pole, and z with the S pole. Do not slide H M along on the needle, just touch the needle as directed.
(C) Cover the needle with filings, then lift it.
EXPERIMENT 18. To study the theory of magnetism.
Apparatus. A thin bar magnet, B M (No. 21); iron filings; a sheet of paper. [Fig. 9] shows simply the edge of B M and the paper. B M should be magnetized as directed in [Exp. 9].
