=70. Directions.= (A) Turn one B M end for end, so that their like poles shall be near each other, but otherwise arranged as in Fig. 19. (B) Make and study the magnetic figure. =EXPERIMENTS 38-39. To study the lifting power of combinations of bar magnets.= _Apparatus for Exps. 38-39._ Two bar magnets, B M (No. 21, 22), of about equal strength; iron filings, I F. =71. Directions.= (A) Find out about how many filings you can lift with the N pole of one magnet. (B) Place the two magnets together (Fig. 20), their _like_ poles being in contact; then see whether the two N poles will lift more or less filings than one pole. [Illustration: Fig. 20.] =EXPERIMENT 39.= =72. Directions.= (A) Remove all filings from the two magnets just used, and hold them tightly together (Fig. 20), with their _unlike_ poles in contact. (B) Compare the amount of filings you can lift at one end of this combination with that lifted in Exp. 38 (A) and (B). _=73. Discussion; Compound Magnets.=_ Many lines of force pass into the air from two like poles. Such a combination is called a _compound magnet_. A piece of thin steel can be magnetized more strongly in proportion to its weight than a thick piece, because the magnetism does not seem to penetrate beyond a certain distance into the steel. Thin steel may be magnetized practically through and through. A thick magnet has but a crust of magnetized molecules; in fact, a thick magnet may be greatly weakened by eating the outside crust away with acid. By riveting several thin bar or horseshoe magnets together, thick permanent magnets of considerable strength are made. _=74.=_ Lines of force, in passing from the N to the S pole of a magnet, meet a resistance in the air, which does not carry or conduct them as easily as iron or steel. In the arrangement of Exp. 39 the lines of force are not obliged to push their way through the air, as each magnet serves as a return conductor for the lines of force of the other. Either magnet may be considered an armature for the other. 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.=