=85. Directions.= (A) Lay B M upon the table and place O C upon its center. Note the position of the compass-needle. (B) Slide O C along from one end of B M to the other, and study the effect upon its needle. Do lines of force curve _over_ B M as well as around its sides, as shown in Exp. 31? (C) Place O C upon the table. Hold B M horizontally above O C, and move O C back and forth under B M. Does the needle remain horizontal, or does it show that lines of force pass _under_ B M on their way from its N to its S pole? [Illustration: Fig. 24.] _=86. The Dip or Inclination of the Magnetic Needle.=_ The needle is said to dip when it takes positions like those in Fig. 24. Compass-needles should be horizontal, when properly balanced, and entirely free from all effects other than those of the earth. The excessive dip shown (Fig. 24) is due, of course, to the efforts of the magnetic needle to place itself in the direction in which the lines of force of B M pass. =EXPERIMENT 46. To study the dip or inclination of the magnetic needle, due to the action of the earth.= _Apparatus._ Fig. 25. Our compass, O C (No. 18); horseshoe magnet, H M (No. 16); piece of paper. =87. Directions.= (A) Place O C upon the table, and mark upon a piece of paper the height of the N pole of its needle above the table. (Fig. 25.) The paper should be held in a vertical position, and near the pole. [Illustration: Fig. 25.] (B) With H M reverse the poles of the compass-needle (Exp. 13), so that its former N pole shall become a S pole. (C) Place the needle upon its pivot again, and mark upon the paper, as before, the height of its new N pole above the table. Does the needle remain horizontal? (D) Remagnetize the needle, and reverse its poles so that it will again balance. [Illustration: Fig. 26.] _=88. Discussion; Balancing Magnetic Needles.=_ If a piece of unmagnetized steel be balanced and then magnetized, it will no longer remain horizontal; it will dip. Try this. Compass-needles are balanced after they are magnetized. Can you now see why the needle did not remain horizontal after its poles were changed? A piece of steel first balanced and then magnetized, has to have its S pole slightly weighted, as suggested by the line at S (Fig. 26 x), to make it horizontal. The magnetic needle does not tend to dip at the earth's equator, because the lines of force of the earth are nearly horizontal at the equator. As we pass toward the north or south on the earth, the lines of force slant more and more as they come from or enter the earth's magnetic poles. What position would the needle take if we should hold it directly over the earth's N magnetic pole? Fig. 24 shows what the needle does when held near the poles of a bar magnet. =EXPERIMENTS 47-48. To study the inductive influence of the earth.=