[Illustration: FIG. 28--WHEN A MAGNET IS THRUST INTO A COIL OF WIRE IT CAUSES A CURRENT TO FLOW IN THE COIL, BUT THE CURRENT FLOWS ONLY WHILE THE MAGNET IS MOVING Drawing reproduced by permission of Joseph G. Branch.] A friend of Faraday, on learning of this discovery, wrote the following impromptu lines: "Around the magnet Faraday Was sure that Volta's lightnings play. But how to draw them from the wire? He took a lesson from the heart: 'Tis when we meet, 'tis when we part, Breaks forth the electric fire." A magnet will produce an electric current in a wire, but only when the magnet or the wire is in motion. Detecting and Measuring an Electric Current The instrument which Faraday used to detect a current was derived from Oersted's experiment. When a current flows in a north-and-south direction over a compass-needle, the needle swings round. When the current stops flowing the needle swings back to the north-and-south position. The effect on the needle is stronger if the current flows through a coil of wire and the coil is placed in a north-and-south position around the needle (Fig. 29). The stronger the current flowing through the coil the farther the needle will turn from the north-and-south position. [Illustration: FIG. 29--A COIL OF WIRE AROUND A COMPASS-NEEDLE The needle tells when a current is flowing, and how strong the current is.] The coil and the needle together are called a galvanometer, and may be used to tell when a current is flowing, and also to indicate the strength of the current. An Electric Current Produced by the Magnetic Field of Another Current Faraday had found that a current flowing around a piece of iron will make the iron a magnet, and that a magnet in motion will cause a current to flow in a wire. It seemed to him that a second wire placed near the first should have a current produced in it without the presence of iron. He wound two coils of copper wire upon the same wooden spool. The wire of the two coils he separated with twine and calico. One coil was connected with a galvanometer, the other with a battery of ten cells, yet not the slightest turning of the needle could be observed. But he was not deterred by one failure. He raised his battery from ten cells to one hundred cells, but without avail. The current flowed calmly through the battery wire without producing, during its flow, any effect upon the galvanometer. During its flow was the time when an effect was expected. Again the unexpected happened. At the instant of making contact with the battery there was a slight movement of the needle. When the contact was broken, another slight movement, but in the opposite direction to the first (Fig. 30). The current in one wire caused a current to flow in the other, but the current in the second wire continued for an instant only at the making and breaking of the contact with the battery. This was the beginning of the induction-coil used to-day in wireless telegraphy.