Suppose _a b c d e f_ to be a part of the wire from A to B, passing above a delicately poised magnetic needle N S, along _a b_ and then below the needle along _c d_, and then above again along _e f_ and so to the station B. Let a current traverse the wire in the direction shown by the arrows. Then N, the north end of the needle, is deflected towards the east by the current passing along _a b_. But it is also deflected to the east by the current passing along _c d_; for this produces a deflection the reverse of that which would be produced by a current in the same direction above the needle—that is, in direction _b a_, and therefore the same as that produced by the current along _a b_. The current along _e f_ also, of course, produces a deflection of the end N towards the east. All three parts, then, _a b_, _c d_, _e f_, conspire to increase the deflection of the end N towards the east. If the wire were twisted once again round N S, the deflection would be further increased; and finally, if the wire be coiled in the way shown in Fig. 1, but with a great number of coils, the deflection of the north end towards the east, almost imperceptible without such coils, will become sufficiently obvious. If the direction of the current be changed, the end N will be correspondingly deflected towards the west. The needle need not be suspended horizontally. If it hang vertically, that is, turn freely on a horizontal axis, and the coil be carried round it as above described, the deflection of the upper end will be to the right or to the left, according to the direction of the current. The needle actually seen, moreover, is not the one acted upon by the current. This needle is inside the coil; the needle seen turns on the same axis, which projects through the coil. If, then, the observer at the station B have a magnetic needle suitably suspended, round which the wire from the battery at A has been coiled, he can tell by the movement of the needle whether a current is passing along the wire in one direction or in the other; while if the needle is at rest he knows that no current is passing. [Illustration: FIG. 2.] [Illustration: FIG. 3.]