The electric telegraph did not come into real use until the magnetic needle was adopted as an indicator. Ten years after the first telegraph was invented, a Danish electrician made a very important discovery, which exercised a far-reaching effect upon electric science. He was trying to discover some connection between electricity and magnetism, and made all sorts of experiments with magnetic compasses. He found at last that a magnetic needle is affected powerfully by a current passing in a wire close by. When a wire bearing an electric current is stretched along a table from north to south, and a magnetic compass is placed on the top of the wire pointing in the same direction, the needle swings round every time the current is sent through the wire, and swings back into its ordinary position as soon as the current is interrupted. So long as the current flows, the needle tends to set itself at right angles to the wire, and will do so almost exactly when the current is very strong. This discovery made a great impression on the scientific world, and many men of science proposed to use it for the purpose of telegraphy. But it was not until the year 1833 that the first telegraph was constructed on this principle. Two professors in Heidelberg, whose names were Gauss and Weber, established a telegraph of this kind between the Physical Institute and the Observatory. The wire had a total length of nearly two miles, and they produced at the receiving station a deflection of a magnetic needle to the right or left. This arrangement was afterwards improved by a young student, who attached pens to two magnetic needles, and made them write dots on a strip of paper passing along under them. Using two different inks, he got different sets of coloured dots for each letter. These could be easily recognised afterwards and read off the paper. The current used in these experiments was not a steady current from a Volta pile or electric battery. It was a so-called induction current. The induction current had been discovered in 1831 by a great Englishman of the name of Michael Faraday, who found that when a wire is placed by the side of another wire through which electricity can pass, then as soon as the current is sent through this wire by connecting it with the battery, a momentary current flows in the opposite direction through the other wire. [Illustration: Fig. 55.--Diagram of Morse System of Telegraphy.] For this to happen, however, it is necessary that the second wire should also form a closed circuit, and not have any free ends. Such an induction current, or induced current, can also be obtained by means of a large magnet. All that is necessary is to take a coil of wire and suddenly slip it upon a bar magnet, or suddenly slip it off. The more rapidly the slipping on or off is done the stronger is the current. In this way, therefore, it is possible to produce an electric current without any piles, or batteries, or chemicals.