The electric current is employed in many diseases, and is of great use in some cases, but the further consideration of it with reference to its medical applications does not fall within the scope of our present work. We will now pass on to one of the most useful applications of the electric force, the Telegraph, and in dealing with it we must make a few remarks upon magnetism. First, let us make an experiment or two, and see the reciprocal action between electricity and magnetism. (1.) If we take a piece of iron of the form of a horse-shoe (fig. 236), and wind around it copper wire, and pass through the wire an electric current from our battery, the iron will exhibit strong magnetic properties, which it will lose when the current is interrupted. The conducting wires are insulated with silk, and the current will then travel in one direction. (2.) If we cover the ends of a non-magnetic piece of iron with coils of wire, and rotate the magnet, A B, so as to cause the poles to approach each end of the iron alternately, an electric current will be established in the wire. (3.) Referring to the first experiment, if we bring a needle in contact with the iron horse-shoe, while the current is passing through the wire we shall find that the needle has become a magnet; _i.e._, that it will point due north and south when suspended. We will now see what a Magnet is, and why it has obtained this name. [Illustration: Fig. 239.—Magnetic attraction.] In Magnesia, in Lydia, in olden times was found a stone of peculiar attributes, which had the property of attracting small portions of iron. The Chinese were acquainted with it, and nowadays it is found in many places. In our childhood we have all read of it in the story of “Sinbad the Sailor.” Popularly it is known as the loadstone; chemists call it magnetic oxide of iron (F_{2}O_{3}). This stone is a _natural magnet_. In Sweden it exists in great quantities as “magnetic iron,” for it has a great affinity for that metal. If we rub a piece of steel upon the loadstone we convert the former into a magnet—an artificial magnet as it is called, and the _magnetic needle_ so useful to us in our compasses and in the working of one form of the electric telegraph is thus obtained. Let us see how this needle acts. [Illustration: Fig. 240.—Simple touch.] [Illustration: Fig. 241.—Double touch.] Take a magnetic needle and dust upon it some iron filings. You will observe that the filings will be attracted to both ends of the magnet, but the centre will remain uncovered. The ends of a magnet are termed “poles,” the centre the equator. So one end is north and the other south, and we might perhaps imagine that the same characteristics would abide in the bar when it is cut in two. But we find that as when a worm is divided, each portion gets a new head or tail, and makes a perfect worm, so in the magnet each divided half becomes a perfect magnet with separate poles, one of which always points to the north.