384. Directions. (A) Arrange the wire so that the current will flow through it from N to S over the compass-needle as soon as the circuit is closed ([Fig. 117], A). Press K for an instant only, and note the direction in which the N pole is deflected. Repeat two or three times until you get clearly in mind the direction taken by the needle. Sketch the result in your note-book, and compare with [Fig. 118], A. The arrow shows the direction of the current.
(B) Let the current pass for an instant from N to S and under the needle, as shown in [Fig. 117], B. Sketch result.
(C) Let the current pass for an instant from S to N above the needle ([Fig. 117], C). Sketch result.
(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].
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.