Professor Henry still further increased the strength of the electromagnet by covering the wire with silk, which made it possible to wind several layers of wire on the iron core, and many times the length of wire that had been used by Sturgeon. Fig. 43 shows such a magnet. One of Henry's magnets weighed fifty-nine and a half pounds, and would hold up a ton of iron. Sturgeon said: "Professor Henry has produced a magnetic force which completely eclipses every other in the whole annals of magnetism." With Professor Henry's invention the electromagnet was ready for use in the dynamo. Fig. 44 shows a strong electromagnet. [Illustration: FIG. 43--AN ELECTROMAGNET WITH MANY TURNS OF INSULATED WIRE] [Illustration: FIG. 44--AN ELECTROMAGNET LIFTING TWELVE TONS OF IRON] A moving magnet causes a current to flow in a coil, but a magnet at rest has no effect. A moving magnet is equal to a battery. In Faraday's experiments a current was induced in a coil of wire by moving a magnet in the coil or by making and breaking the circuit in another coil wound on the same iron core. A current was induced in a metal disk by revolving it between the poles of a magnet. In every case there was motion in a magnetic field, or the field itself was changed. A changing magnetic field is equal to a moving magnet. What is needed to induce a current in a coil, whether it be in a dynamo, an induction-coil, or a transformer, is a changing magnetic field about the coil or motion of the coil in the magnetic field. If fine iron filings are sprinkled over the poles of a magnet the filings arrange themselves in definite lines. This is a simple experiment which any boy can try for himself. Faraday called the lines marked out by the iron filings "lines of force" (the lines of force of a horseshoe magnet are shown in Fig. 36), because they indicate the direction in which the magnet pulls a piece of iron--that is, the direction of the magnetic force. Now, if a current is to be induced in a wire, the wire must move across the lines of force. If the wire moves along the lines marked out by the iron filings, there will be no current. When a coil rotates between the poles of a magnet, the wire moves across the lines of force and a current is induced in the coil if the circuit is closed. This is the way a current is produced in a dynamo. Faraday produced a current by rotating a coil between the poles of a steel magnet. He made a number of such machines, and used them with some success in producing lights for lighthouses, but the defects of these machines were so great that the lighting of a city or the development of power on a large scale was impractical. The electromagnet was needed to solve the problem. Siemens' Dynamo