If we should examine the space in the immediate neighborhood of a coil of wire carrying a current of electricity it would be found that a similar state of affairs existed there and that the coil also possessed a magnetic field composed of lines of force flowing around it.
This is readily shown by punching a small hole in a piece of cardboard and passing a wire carrying a current of electricity through the hole. If iron filings are sprinkled on the cardboard, they will arrange themselves in circles around the wires, forming a magnetic phantom and showing that a coil of wire carrying a current of electricity generates a magnetic field in its vicinity. By forming the wire into a coil the magnetic field generated is much stronger, for the then combined effect of the wires is secured.
FIG. 32.—Magnetic phantom formed by wire carrying current.
FIG. 33.—Magnetic phantom formed by coil of wire carrying current.
The induction coil and transformer are simply instruments utilizing the principle that a coil of wire carrying a current possesses a magnetic field which will induce a current of electricity in another neighboring coil.
FIG. 34.—Diagram of an induction coil.
The induction coil consists essentially of a primary winding of heavy wire wound around a soft iron core and surrounded by a secondary coil consisting of many thousand turns of fine wire, carefully insulated. The current from a battery is sent through the primary coil and sets up a magnetic field. The magnetic field induces a current in the secondary whose voltage is approximately proportional to the ratio of the turns of the secondary to the primary. Thus, if the secondary contains one hundred times as many turns of wire as the primary the induced voltage will be one hundred times the voltage of the original primary current. The purpose of the iron core is to concentrate the magnetic field and make the coil more efficient. Since currents are only induced in the secondary when the magnetic field is changing, an automatic device called an interrupter or sometimes a vibrator, is employed to rapidly turn the current flowing through the primary on and off. The interrupter consists of a spring carrying a platinum point against which presses a second piece of platinum on the end of an adjustable thumbscrew. Platinum is necessary because the current of electricity would quickly oxidize and burn up any other material. The interrupter spring is placed near the end of the core so that the magnetism of the latter will draw it forward away from the thumbscrew and interrupt the current. As soon as the current ceases to flow the core loses its magnetism and the spring returns to its former position repeating the cycle very rapidly a large number of times per second. The interrupter is fitted with a condenser shunted across its terminals to stop sparking at the platinum points and also to make the currents in the secondary more intense.