It takes force to move a magnet through the center of a coil, and it is this work that is the source of the induced current. We have, in this simple experiment, the key to the action of the dynamo and other electrical machines. =102. Current from two Coils.= Fig. 101 shows two coils of wire, the smaller being connected to a cell, the larger to a galvanometer. By moving the small coil up and down inside of the large one, induced currents are generated, first in one direction and then in the opposite. We have here two entirely separate circuits, in no way connected. The _primary_ current comes from the cell, while the _secondary_ current is an induced one. By placing a core in the small coil of Fig. 101, the induced current will be greatly strengthened. It is not necessary to have the two coils so that one or both of them can move. They may be wound on the same core, or otherwise arranged as in the induction coil. (See "Study," Chapter XXV., for experiments on induced currents.) CHAPTER XIII. HOW THE INDUCTION COIL WORKS. =103. The Coils.= We saw, § 102, that an induced current was generated when a current-carrying coil, Fig. 101, was thrust into another coil connected with a galvanometer. The galvanometer was used merely to show the presence of the current. The _primary coil_ is the one connected with the cell; the other one is called the _secondary coil_. [Illustration: Fig. 102.] When a current suddenly begins to flow through a coil, the effect upon a neighboring coil is the same as that produced by suddenly bringing a magnet near it; and when the current stops, the opposite effect is produced. It is evident, then, that we can keep the small coil of Fig. 101 with its core inside of the large coil, and generate induced currents by merely making and breaking the primary circuit. We may consider that when the primary circuit is closed, the lines of force shoot out through the turns of the secondary coil just as they do when a magnet or a current-carrying coil is thrust into it. Upon opening the circuit, the lines of force cease to exist; that is, we may imagine them drawn in again. =104. Construction.= Fig. 102 shows one form of home-made induction coil, given here merely to explain the action and connections. Nearly all induction coils have some form of automatic current interrupter, placed in the primary circuit, to rapidly turn the current off and on. _Details of Figs. 102 and 103._ Wires 5 and 6 are the ends of the primary coil, while wires 7 and 8 are the terminals of the secondary coil. The primary coil is wound on a bolt which serves as the core, and on this coil is wound the secondary which consists of many turns of fine wire. The wires from a battery should be joined to binding-posts W and X, and the handles, from which the shock is felt, to Y and Z. Fig. 103 shows the details of the interrupter. [Illustration: Fig. 103.]