[Illustration: FIG. 30--FARADAY'S INDUCTION-COIL Starting and stopping the battery current in the primary coil causes a changing magnetic field, and this causes a current to flow in the secondary coil. Drawing reproduced by permission of Joseph G. Branch.] What was the secret of it? Simply this: that a current in one wire will cause a current to flow in another wire near it, but only while the current in the first wire is changing. That is, at the instant when the first wire is connected to the battery, or its connection broken, a current is induced in the second wire. There is no battery or other source of current connected to the second wire; but a current flows in this wire because it is near a wire in which a current is rapidly starting and stopping. When these two wires are wound in coils, together they form an induction-coil. The wire which we have called the first wire forms the "primary" coil, and the one we have called the second wire forms the "secondary" coil. By repeatedly making and breaking the circuit in the primary coil we get an alternating current in the secondary coil. Fig. 31 is from a photograph of some of the coils actually used by Faraday. [Illustration: FIG. 31--HISTORICAL APPARATUS OF FARADAY IN THE ROYAL INSTITUTION Some of Faraday's transformer coils are shown here. The instrument on the left in a glass case is his galvanometer.] Faraday's Dynamo To invent a new electrical machine was Faraday's next aim. Arago's disk of copper whirling near a magnet had a current induced in it, so Faraday thought. It was the action of this induced current which caused the magnet to follow the whirling disk. Could the current in Arago's disk be collected and caused to flow through a wire? He placed a copper disk between the poles of a magnet. One galvanometer wire passed around the axis of the disk, the other he held in contact with the edge. He whirled the disk. The galvanometer needle moved. A current was flowing in the disk as it whirled. The current from the whirling disk flowed through the galvanometer. Faraday had discovered the dynamo (Fig. 32). [Illustration: FIG. 32--FARADAY'S FIRST DYNAMO A current flows in the copper disk as it whirls between the poles of the magnet. By permission of Joseph G. Branch.] All this work occupied but ten days in the autumn of 1831, though years of preparation had gone before. In these ten days the foundation was laid for the induction-coil, modern dynamo-electric machinery, and electric lighting. Fig. 33 shows the laboratory in which Faraday did this work. [Illustration: FIG. 33--FARADAY'S LABORATORY, WHERE THE FIRST DYNAMO WAS MADE From the water-color drawing by Miss Harriet Moore.]