=410. Directions.= (A) Arrange as in Exp. 164, but reverse the direction of the current through the coils. Do this by joining O of the key (Fig. 126) with R of Fig. 132, and Zn of the cell with L. (B) Repeat (B) and (C) of Exp. 164 and study results. [Illustration: Fig. 133.] =EXPERIMENT 166. To test the poles.= =411. Directions.= (A) Arrange all connections as in Exp. 164, then reverse the positions of O E and I E of coil A; that is, join O E to M, and I E to L, Fig. 132. This will make unlike ends come together at M; in other words, when the current enters at L and leaves at R it will pass around both coils in the same direction. (B) Study the nature of the poles, as in Exps. 164, 165, and note results. _Note._--Fig. 133 shows simply the two cores of a horseshoe electromagnet with arrows to indicate in which direction the current is passing in each coil to produce N and S poles. =EXPERIMENT 167. To study the inductive action of one core upon the other.= =412. Directions.= (A) Arrange as for Exp. 164, but join the wire from Zn of the cell to M (Fig. 132). In this way coil B will be cut out of the circuit. Place the coils in the E and W line. (B) Find about how far the residual magnetism of the core of B can act upon the compass-needle, holding the compass on the side away from coil A, no current passing. (C) Press the key for an instant, and note whether the magnetism of coil B has been made stronger or weaker. Explain the action of core A on core B. =EXPERIMENT 168. Magnetic figures.= =413. Directions.= (A) Arrange as in Exp. 164. With books, etc., fix a piece of smooth, stiff paper, or a sheet of glass, just above the poles of the electromagnets. (B) Sprinkle iron filings upon the glass, and gently tap it while the circuit is closed at the key for a few seconds. Make a sketch of the magnetic figure produced. Do the lines of force from the opposite poles attract or repel each other? See § 417 for making permanent figures. (See "Things a Boy Should Know About Electricity" for drawings of magnetic figures.) _Note._--If possible, use two or three good cells in series for making magnetic figures, as a fairly strong field is best. =EXPERIMENT 169. Magnetic figures.= =414. Directions.= (A) Arrange apparatus as for Exp. 165, and make the magnetic figure for this combination, as directed in Exp. 168. Sketch and study the results. =EXPERIMENT 170. Magnetic figures.= =415. Directions.= (A) Arrange the apparatus and connections as in Exp. 166, and make the magnetic figure of this combination as directed in Exp. 168. In this case the poles are alike. Sketch and study the results. =EXPERIMENT 171. Magnetic figures.=