=400. Directions.= (A) Join the electromagnet with the cell and key as before (Exp. 154). Hold one end of the core firmly against the top of a tin box which should stand upon the table and which should act as a sounding-board. The flat boxes used in the experiments on static electricity are good for this, or use the tin box, No. 94, for a base. Rapidly open and close the circuit by means of the key and listen for any clicks made by the core. (B) Listen for this sound in telegraph sounders, electric bells, etc., if you have them. The armature should be held, of course, so that slight sounds can be heard. _=401. Discussion.=_ A bar of iron becomes slightly longer when it is magnetized, the particles of iron being made to point in the same direction. As soon as the current ceases to flow through the coil the particles of the soft core nearly all resume their mixed positions. The click heard is supposed to be due to the changes in the molecules of iron. The core becomes gradually warmer when it is rapidly magnetized and demagnetized by a strong current. [Illustration: Fig. 126.] =EXPERIMENT 161. Magnetic figures.= =402. Directions.= (A) Arrange as in Fig. 126. The key should be used in case a dry cell acts as the source of the current. Two good cells joined in series can be used to advantage. Lay the coil flat upon the table and place on it a piece of stiff, smooth paper, or a sheet of glass. (B) Sprinkle a few iron filings upon the glass, which may be held in place by books. Gently tap the glass with a pencil while you close the circuit at the key. Do the filings arrange themselves as in the case of permanent magnets? Make a sketch of the field, remembering that you have both N and S poles, and compare it with previous results. [Illustration: Fig. 127.] =EXPERIMENT 162. Magnetic figures.= =403. Directions.= (A) Arrange as in Fig. 126, but stand the coil on end, using the base as directed in § 407, to hold it firmly in position. Join the ends, O E and I E, to the key as before. Fig. 127 shows a top view of the coil and base. (B) With books, etc., fix a piece of stiff, smooth paper, or glass just over the top of the core, and proceed as in Exp. 161 to study the field. See § 417 for making permanent pictures of magnetic fields. =EXPERIMENT 163. Magnetic field.= =404. Directions.= (A) Use same arrangement as for Exp. 162, except filings and glass, which are replaced by the compass. (B) Hold the compass about 2 in. from the top pole of the electromagnet, close the circuit for a second or two and note action of needle. Is the top N or S, when the current enters the coil at O E? Compare result with § 392.