Oersted discovered in 1820 that a magnetic needle poised horizontally is deflected when the galvanic current passes above it (parallel to the needle’s length) or below it. If the current passes above it, the north end of the needle turns towards the east when the current travels from north to south, but towards the west when the current travels from south to north; on the other hand, if the current passes below the needle, the north end turns towards the west when the current travels from south to north, and towards the east when the current travels from north to south. The deflection will be greater or less according to the power of the current. It would be very slight indeed in the case of a needle, however delicately poised, above or below which passed a wire conveying a galvanic current from a distant station. But the effect can be intensified, as follows:—
Fig. 1.
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.
Fig. 2.
Fig. 3.