The current will pass along any length of conducting substance properly insulated. Suppose, then, an insulated wire passes from the positive pole of a battery at a station A to a station B, and thence back to the negative pole at the station A. Then the current passes along it, and this can be indicated at B by some action such as electricity of low intensity can produce. If now the continuity of the wire be interrupted close by the positive pole at A, the current ceases and the action is no longer produced. The observer at B knows then that the continuity of the wire has been interrupted; he has been, in fact, signalled to that effect. But, as I have said, the electrical phenomena which can be produced by the current along a wire connecting the positive and negative poles of a galvanic battery are not striking. They do not afford effective signals when the distance traversed is very great and the battery not exceptionally strong. Thus, at first, galvanic electricity was not more successful in practice than frictional electricity. It was not until the effect of the galvanic current on the magnetic needle had been discovered that electricity became practically available in telegraphy. 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:— [Illustration: FIG. 1.]