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.
Oersted’s discovery of the production of magnetism by electricity naturally suggested the possibility of producing electricity from magnetism. In the year 1831 one of the most brilliant of our British scientists, Michael Faraday, discovered that a current of electricity could be induced in a coil of wire either by moving the coil towards or away from a magnet, or by moving a magnet towards or away from the coil. This may be shown in a simple way by connecting the ends of a coil of insulated wire to a galvanometer, and moving a bar magnet in and out of the coil; when the galvanometer shows that a current is induced in the coil on the insertion of the magnet, and again on its withdrawal. We have seen that a magnet is surrounded by a field of magnetic force, and Faraday found that the current was induced when the lines of force were cut across.
Utilizing this discovery Faraday constructed the first dynamo, which consisted of a copper plate or disc rotated between the poles of a powerful horse-shoe magnet, so as to cut the lines of force. The current flowed either from the shaft to the rim, or vice versa, according to the direction of rotation; and it was conducted away by means of two wires with spring contacts, one pressing against the shaft, and the other against the circumference of the disc. This machine was miserably inefficient, but it was the very first dynamo, and from it have been slowly evolved the mighty dynamos used to-day in electric power stations throughout the world. There is a little story told of Faraday which is worth repeating even if it is not true. Speaking of his discovery that a magnet could be made to produce an electric current, a lady once said to him, “This is all very interesting, but what is the use of it?” “Madam,” replied Faraday, “what is the use of a baby?” In Faraday’s “baby” dynamo, as in all others, some kind of power must be used to produce the necessary motion, so that all dynamos are really machines for converting mechanical energy into electrical energy.