_=387.=_ If a wire carrying a strong current be dipped in iron filings, the magnetic field about the wire acts by induction upon the particles of filings, making magnets of them. These cling to each other simply because they are little magnets.
_=388. Lines of Force about Parallel Wires.=_ When a current passes in the same direction in two parallel wires the lines of force pass around the wires in the same direction in both, and the magnetic fields attract each other. When the currents flow in opposite directions the magnetic fields repel each other.
=EXPERIMENT 153. To study the lines of force about a coil of wire like that upon the galvanoscope.=
_Apparatus._ Galvanoscope, G V; dry cell; key; compass. Arrange as in Fig. 116, using G V instead of the compass shown. The coil of G V should be placed in the E and W line. The current can pass only when the key is pressed. Connect the wires with G V, so that the current will pass through the 15-turn coil from W to E on top of the coil; that is, so that the current will have a "clockwise" motion. Fig. 120 represents a front view of the coil.
=389. Directions.= (A) Hold the compass in the various places marked with a dot (Fig. 120) and note the directions taken by its N pole. Make a circle similar to the one shown to represent the coil, and sketch upon it the way in which the lines of force pass around it according to your observations.
(B) Make a diagram like Fig. 121, which represents a cross-section of the coil through the center. Imagine that you have removed the top half of the coil and that you are looking down upon the ends of the wire of the lower half. Draw curved arrows about the coil at W and E to show which way the lines of force are passing. Compare your results with those in Fig. 119, remembering that at E, Fig. 121, the current is going away from you.
(C) Move O C back and forth on the center-line that runs N and S through the coil, and note the positions of the compass-needle. Does the coil seem to have poles?
(D) Reverse the current through the coil and repeat your observations.
=EXPERIMENT 154. To study the magnetic field about a small coil of wire.=
_Apparatus._ A coil of wire (No. 89), described in § 390; current reverser, C R (No. 57); dry cell; connecting wires, etc.