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_.

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.

If the current from a cell enters at W, it will pass through the primary coil and out at X, after going through 5, R, F, S I, B, E and C. The instant the current passes, the bolt becomes magnetized; this attracts A, which pulls B away from the end of S I, thus automatically opening the circuit. B at once springs back to its former position against SI, as A is no longer attracted; the circuit being closed, the operation is rapidly repeated.

A _condenser_ is usually connected to commercial forms. It is placed under the wood-work and decreases sparking at the interrupter. (See "Apparatus Book," Chapter XI., for home-made induction coils.)

Fig. 104 shows one form of coil. The battery wires are joined to the binding-posts at the left. The secondary coil ends in two rods, and the spark jumps from one to the other. The interrupter and a switch are shown at the left.

Fig. 105 shows a small coil for medical purposes. A dry cell is placed under the coil and all is included in a neat box. The handles form the terminals of the secondary coil.

=105. The Currents.= It should be noted that the current from the cell does not get into the secondary coil. The coils are thoroughly insulated from each other. The secondary current is an induced one, its voltage depending upon the relative number of turns of wire there are in the two coils. (See Transformers.) The secondary current is an alternating one; that is, it flows in one direction for an instant and then immediately reverses its direction. The rapidity of the alternations depends upon the speed of the interrupter. Coils are made that give a secondary current with an enormous voltage; so high, in fact, that the spark will pass many inches, and otherwise act like those produced by static electric machines.

=106. Uses of Induction Coils.= Gas-jets can be lighted at a distance with the spark from a coil, by extending wires from the secondary coil to the jet. Powder can be fired at a distance, and other things performed, when a high voltage current is needed. Its use in medicine has been noted. It is largely used in telephone work. Of late, great use has been made of the secondary current in experiments with vacuum-tubes, X-ray work, etc.