(D) Let it pass from S to N _under_ the needle (Fig. 117, D). Sketch result. (E) Let it pass through the wire from east to west (Fig. 117, F) above the needle, then under it, and note result. Compare the results with those indicated in Fig. 118. [Illustration: Fig. 118.] [Illustration: Fig. 119.] _=385. Lines of Force About a Wire.=_ When a current passes through a wire, the needle, over or under it, tends to take a position at right angles to the wire. This shows that the lines of force pass _around_ the wire and not in the direction of its length. The needle does not swing entirely perpendicular to the wire; that is, to the E and W line, because the earth is at the same time pulling its N pole towards the N. If the needle had no pointing power, and at the same time retained its magnetic field, it would point exactly at right angles to the wire as soon as the current passed. If you look along the wire, Fig. 119, from the point, C, towards the positions, A and B, you will see (A) that _under_ the wire the lines of force pass to the left, and that _above_ the wire (B) they pass towards the right. This is because the N pole points in the directions mentioned. (See Fig. 118.) Looking along the wire from Z towards position, D and C, you will see just the opposite to the above, as the current comes _towards_ you. _Rule._--When the current goes from you, the lines of force pass around the wire in a clockwise direction, and when the current comes toward you they pass around it in an anti-clockwise direction. _=386. Ampere's Rule=_ may be used to remember what has been learned in Exp. 152. _If you imagine yourself swimming in the wire with the current, always facing the needle, the N-seeking pole of the needle will always be deflected towards your left hand._ When the needle is above the wire you must imagine that you swim upon your back, in order to _face_ the needle. _Another Rule._--Hold the right hand with the thumb extended and with the fingers pointing in the direction of the current, the palm being towards the needle and on the opposite side of the wire from the needle. The N-seeking pole will then be deflected in the direction in which the thumb points. _=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.=