The efficiency of the coil is greatly increased by a condenser which is inserted in the primary circuit. It consists of alternate layers of tinfoil and paraffined paper, and its action is like that of a Leyden jar. A switch is provided to turn the battery current on or off, and there is also a reversing switch or commutator, by means of which the direction of the current may be reversed. The whole arrangement is mounted on a suitable wooden base, and its general appearance is shown in Fig. 18. [Illustration: _By permission of_] [_Harry W. Cox, Ltd._ FIG. 18.--Typical Induction Coil.] By means of a large induction coil we can obtain a voltage hundreds or even thousands of times greater than that of the original battery current, but on account of the great resistance of a very long, thin wire, the amperage is much smaller. The induction coil produces a rapid succession of sparks, similar to those obtained from a Wimshurst machine. A coil has been constructed capable of giving sparks 42½ inches in length, and having a secondary coil with 340,000 turns of wire, the total length of the wire being 280 miles. Induction coils are largely employed for scientific purposes, and they are used in wireless telegraphy and in the production of X-rays. The principle of the induction coil can be applied also to the lowering of the voltage of a current. If we make the secondary coil with less, instead of more turns of wire than the primary coil, the induced current will be of lower voltage than the primary current, but its amperage will be correspondingly higher. This fact is taken advantage of in cases where it is desirable to transform a high voltage current from the public mains down to a lower voltage current of greater amperage. CHAPTER IX THE DYNAMO AND THE ELECTRIC MOTOR Most of my readers will have seen the small working models of electric tramcars which can be bought at any electrical supply stores. These usually require a current of about one ampere at three or four volts. If we connect such a car to the battery recommended for it, and keep it running continuously, we find that the battery soon begins to show signs of exhaustion. Now if we imagine our little car increased to the size of an electric street car, and further imagine, say, a hundred such cars carrying heavy loads day after day from morning to night, we shall realize that a battery of cells capable of supplying the current necessary to run these cars would be so colossal as to be utterly impracticable. We therefore must look beyond the voltaic cell for a source of current for such a purpose, and this source we find in a machine called the “dynamo,” from the Greek word _dynamis_, meaning force.