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.) [Illustration: Fig. 104.] 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. [Illustration: Fig. 105.] =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. CHAPTER XIV. THE ELECTRIC TELEGRAPH, AND HOW IT SENDS MESSAGES. =107. The Complete Telegraph Line= consists of several instruments, switches, etc., etc., but its essential parts are: The _Line_, or wire, which connects the different stations; the _Transmitter_ or _Key_; the _Receiver_ or _Sounder_, and the _Battery_ or _Dynamo_. =108. The Line= is made of strong copper, iron, or soft steel wire. To keep the current in the line it is insulated, generally upon poles, by glass insulators. For very short lines two wires can be used, the line wire and the return; but for long lines the earth is used as a return, a wire from each end being joined to large metal plates sunk in the earth. [Illustration: Fig. 106.]