Magnetism seems to reach out in lines from the poles of a magnet. The position and direction of some of the lines are shown by the lines of filings. They are very distinct near the poles, and are considered, for convenience, to start from the N pole of a magnet, where they separate. They then pass through the air on all sides of the magnet, and finally enter it again at the S pole. These lines are called _lines of force_ or _lines of magnetic induction_. The poles must not be considered mere points at the ends of a magnet. As shown by magnetic figures, the lines of magnetic induction flow from a considerable portion of the magnet's ends. =EXPERIMENTS 33-37. To study the magnetic fields of various combinations of bar magnets.= _Apparatus for Exps. 33-37._ Two bar magnets, B M (Nos. 21, 22); an iron ring, I R (No. 23); iron filings, I F; a sheet of stiff paper; the sifter (No. 24). =65. Note.= The student will find it very helpful to make the magnetic figures of the combinations given. Thoroughly magnetize the bar magnets upon an electro-magnet, or upon a strong horseshoe magnet, and mark their N poles in some way. The N poles may be marked by sticking a small piece of paper to them. =66. Directions.= (A) Arrange the two magnets, B M, as in Fig. 18, with their unlike poles about an inch apart. (The dotted circle indicates the iron ring to be used in the _next_ experiment. About a quarter, only, of the magnets are shown.) (B) Place the paper over the magnets, and sift filings upon it immediately over the unlike poles. Note particularly the lines of filings between N and S. (C) Make a sketch of the result. (See experiments with electromagnets, and the illustrations of magnetic figures with them.) =EXPERIMENT 34.= =67. Directions.= (A) Leaving the opposite poles an inch apart, as in Exp. 33, place the iron ring, I R (No. 23), between them (Fig. 18, dotted circles). (B) Place the paper over it all, and sprinkle filings upon it to get the magnetic figure. (C) Make a sketch of the resulting figure, and compare it with the figure made in Exp. 33. Why do the lines of force appear indistinct in the center of the ring and around it? (See §74.) [Illustration: Fig. 18.] [Illustration: Fig. 19.] =EXPERIMENT 35.= =68. Directions.= (A) Arrange the two bar magnets, as in Exp. 33, but with their two N poles an inch apart. (B) Make the magnetic figure of the combination. Do the lines of force flow from one N pole directly to the N pole of the other? Do the particles of filings reaching out from one B M attract or repel those from the other B M? =EXPERIMENT 36.= =69. Directions.= (A) Place the two bar magnets side by side, so that their unlike poles shall be arranged as in Fig. 19. (B) Make the magnetic figure. =EXPERIMENT 37.=