THE TELEPHONE
You probably all know that the telephone is an electrical instrument by which one person may talk to another who is at a distance. Not only can we talk to a person who is in a different part of the city, but such great improvements have been made in these instruments that we can talk through the telephone to a person in another city, even though it be hundreds of miles away.
The main principle of the telephone is electromagnetism, as in the telegraph, but there are other important points in addition to those we mentioned in describing the latter.
Let us take first the
INDUCTION-COIL
You will remember that an electromagnet is made by winding many turns of wire around a piece of iron and sending a current of electricity through this wire.
Now, suppose this current of electricity was being supplied by two cells of a battery. If you took in your hands the wires coming from these two cells, giving, say, four volts, you could not feel any shock; but if you were to take hold of the ends of the wires on the electromagnet and separate them while this same current was going through, you would get a decided shock.
This separation would "break" the circuit, and the reason you would get a shock is that, while the electricity is acting on the wire, the iron itself is magnetized, and on breaking the circuit reacts upon the wire, producing for a moment more volts of pressure in every turn of it. Thus, you see, this weak pressure of electricity as it travels through the wire can yet produce, through its magnetism, strong momentary effects, but you cannot feel it unless you break the circuit.
HOW THE INDUCTION-COIL IS MADE
The object of the induction-coil is to produce high intensity, or pressure, from a comparatively weak pressure and large current of electricity; so, if we add still more wire, the magnet has a larger number of turns to act upon and thus makes a very strong pressure, or large number of volts, but a lesser number of ampères.