Figure 92 shows a circuit for connecting two instruments on a line. When the double contact switch is on point A the batteries are out of circuit. Throwing the switch on B puts the batteries into circuit. They should be kept out of circuit when the stations are not in use. The circuit closer at Station 1 must be kept closed when Station 2 is telegraphing and vice versa. CHAPTER IX. HOW TO MAKE AND INSTALL A TELEPHONE. Not many years ago, the telephone was the wonder of the times just as the aeroplane and the wireless are to-day. The original telephone apparatus was an exceedingly crude and simple arrangement which has gradually developed into a wonderful and complex system which makes it possible to carry on a conversation with almost any other point in the United States as easily as it is face to face. [Illustration: FIG. 93.—The Wooden Back for the Telephone.] Many of my young readers have undoubtedly already constructed a set of telegraph instruments and rigged up a line with some nearby chum. Like the telegraph, the principle of the telephone is that of a current of electricity flowing over a line wire into a pair of electro-magnets, and it is not any more difficult to install a short telephone line than a telegraph. Of course there are many important differences between a telephone and a telegraph but they are not intricate or complex. The telephone is exceedingly sensitive when compared to the telegraph. An ordinary telegraph relay requires about one hundredth of an ampere to operate it, whereas a telephone receiver will produce an audible sound on a current of less than one millionth of an ampere. [Illustration: FIG. 94.—The Complete Telephone.] The telephone may be defined as an apparatus for transmitting speech to a distance by means of electricity. The part of the apparatus which takes up the sounds and changes them into electric currents is the transmitter. Where words are spoken into the mouth-piece of the transmitter, they strike a thin metal diaphragm, to the back of which is fastened a small cup-shaped piece of carbon. A second cup is mounted in a rigid position directly back of the first and the space between is filled with small polished granules of carbon. When the granules are undisturbed and are lying loosely together their resistance to an electric current is so great that they allow almost none to flow. When the vibrations of the voice strike the diaphragm they move it back and forth and cause the carbon cup mounted on its back to move and exert a varying pressure on the granules. When the granules are compressed their resistance is greatly lowered and they permit current to pass. That part of the apparatus which transforms the pulsating current back into sound waves is called the receiver. It consists of a thin iron disk, placed very near but not quite touching the end of a small permanent magnet about which is wound a coil of fine insulated wire. [Illustration: FIG. 95.—Details of the Receiver Hook.]