FIG. 28.—Diagram showing how batteries may be arranged in "series" or "series multiple."
FIG. 29.—The power plant of a Marconi transatlantic station, showing engine and generator.
When the source of current supply is alternating, an induction coil may be operated as a transformer. Both induction coils and transformers are instruments for raising the voltage of the ordinary available current from a comparatively low value, 6-220 volts, to a quantity (15,000–20,000 volts), where it can properly charge the aerial and create a state of strain, or, as it is called in technical parlance, an electro-static field.
FIG. 30.—If a magnet is suddenly plunged into a hollow coil of wire a momentary electric current will be induced in the coil.
Both the induction coil and transformer depend for their operation upon the principles of magnetic induction. In 1831, Michael Faraday, a famous English chemist and physicist, discovered that if a magnet be suddenly plunged into a hollow coil of wire, that a momentary current of electricity is generated in the coil. As long as the magnet remains motionless, it induces no current in the coil, but when it is moved back and forth, it sets up the currents. The source of electrical energy is the mechanical work done in moving the magnet. The medium which changes the mechanical energy into electricity is called the magnetic field. The magnetic field is a peculiar state or condition of the space in the immediate neighborhood of a magnet. Its real nature is very hard to explain and not easily understood. Suffice it to say, however, that the current is induced in the coil of wire only when the magnetic field is changing, either decreasing or increasing. The change is produced by moving the magnet because its influence on the coil will be great or small accordingly as it is near or far.
FIG. 31.—Magnetic phantom formed by bar magnet.
It is possible to show the existence of the magnetic field by placing a sheet of glass over a bar magnet and then sprinkling iron filings on the glass. They will settle down in curving lines as in Fig. 31, forming a magnetic phantom. The curved lines formed by the filings represent the direction of the lines of force which make up the magnetic field.