the other coil and away to the battery. Now, a coil through which a current passes becomes a magnet. Its polarity depends on the direction in which the current flows. Suppose that you are looking through the coil, and that the current enters it from your end. If the wire is wound in a clockwise direction, the S. pole will be nearest you; if in an anti-clockwise direction, the N. pole. In Fig. 55 the N. poles are at the right end of the coils, the S. poles at the left end; so the N. pole of the needle is attracted to the right, and the S. pole to the left. When the current is reversed, as in Fig. 56, the needle moves over. If no current passes, it remains vertical.
METHOD OF REVERSING THE CURRENT.
A simple method of changing the direction of the current in a two-instrument circuit is shown diagrammatically in Fig. 57. The principle is used in the Wheatstone needle instrument. The battery terminals at each station are attached to two brass plates, A B, A 1 B 1 . Crossing these at right angles (under A A 1 and over B B 1 ) are the flat brass springs, L R, L 1 R 1 , having buttons at their lower ends, and fixed at their upper ends to baseboards. When at rest they all press upwards against the plates A and A 1 respectively. R and L 1 are connected with the line circuit, in which are the coils of dials 1 and 2, one at each station. L and R 1 are connected with the earth-plates E E 1 . An operator at station 1 depresses R so as to touch B. Current now flows from the battery to B,