_=35. Discussion; Reversal of Poles.=_ The poles of weak magnets may be easily reversed. This often occurs when the apparatus is mixed together. It is always best, before beginning an experiment, to remagnetize the pieces of steel which have already served as magnets. The same may be shown by magnetizing a needle, rubbing it first in one direction, and then in another upon the magnet, testing, in each case, the poles produced. =EXPERIMENT 15. To find whether we can make a magnet with two N poles.= _Apparatus._ The horseshoe magnet, H M; an unmagnetized sewing-needle; compass, O C (No. 18). =36. Note.= You have already learned that the polarity of a weak magnet can be changed (Exp. 14). Can you think of any method by which _two N poles_ can be made in one piece of steel? =37. Directions.= (A) Place the needle upon H M, as in Fig. 7. (B) Keeping the part, C, in contact with the N pole of H M, and using the N pole of H M as a pivot, turn the needle end for end so that its head will be in contact with the S pole of H M. (C) Pull the needle straight from H M, being careful not to slide it in either direction. (D) Test the polarity of the ends with O C (Fig. 5), and save it for the next experiment. [Illustration: Fig. 7.] [Illustration: Fig. 8.] =EXPERIMENT 16. To study the bar magnet with two N poles.= _Apparatus._ The strange magnet just made (Exp. 15); iron filings; compass, O C (No. 18). =38. Directions.= (A) Sprinkle filings over the whole length of the needle and then raise it (Fig. 8). (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.=