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.]
If the transmitter and receiver are connected together in series with a battery and words are spoken into the transmitter, the little carbon granules are immediately thrown into motion, and being alternately compressed and released cause corresponding changes in the current flowing through the receiver from the battery.
The strength of the magnetic pull exerted by the combined permanent magnet and coil of wire varies with each change in the electric current and thus by alternately attracting and repelling the diaphragm causes it to vibrate and emit sounds.
This is the general principle of the telephone. The telephones in actual service to-day are complicated with bells, induction coils, condensers and relays, which render them more efficient, but the general principle of the transmission of the speech is the same.
I would not advise the young experimenter to attempt to build his own transmitter and receiver except for the knowledge and experience gained thereby. Homemade instruments of this sort are not likely to prove sensitive enough for practical purposes over a long line.
A first class telephone receiver can be purchased for 90 cents and an equally good transmitter for $2.25 These, together with an ordinary electric bell can be built into a first class telephone instrument which will give excellent service over a long line.
[Illustration: FIG. 96.—Showing how the Push Button is arranged.]
Any boys can easily build and install such an arrangement. When completed it will prove invaluable as a source of communication from one part of the house to another, from the house to the barn or between two chums living in the same neighborhood. Figure 93 shows the wooden back of the instrument. It is made from a piece of board 12 inches long and 7 inches wide, by cutting into the shape shown. This back serves to mount the transmitter, receiver, bell, etc.
*The Bell* is for the purpose of giving an audible signal so that the person hearing it will know that some one at the other end of the line wishes, to talk.
Figure 94 is a front view of the completed instrument. The bell is mounted on the upper part of the back as shown in the illustration.
*The Receiver Hook* is shown in detail in Figure 95. It should be cut out of heavy sheet brass and finished in the shape shown.
The receiver hook is mounted directly under the bell as shown in Figure 94. The hook should project out to the left beyond the edge of the back. The hook is mounted by a round headed brass screw passing through the hole in the right hand end.
[Illustration: FIG. 97.—Circuit showing how to connect two Telephone
Stations to the Line.]
The hook should move freely up and down between two contacts (marked 1 and 2 in Figure 94). These contacts are simply two small brass strips bent at right angles and fastened about an inch apart along the left hand edge of the wooden back by means of two small round headed brass wood screws. The position of the contacts should be such that the hook is in a horizontal position when it is half way between the two. A small spring is arranged so as to pull the hook up against the upper contact (No. 1) when the receiver is lifted off. When the receiver is hung on the hook, it should overcome the lift of the spring and pull the hook down against the lower contact (No. 2).
*The Receiver* may be purchased at almost any reliable electrical supply house. It should be provided with a ring so that it can be hung onto the hook.
*The Transmitter* is preferably of the "solid back" long distance type. It also can be purchased ready made from almost any electrical supply house.
The transmitter is mounted on the wooden back directly under the hook.
It is marked T in Figure 94.
*A Push Button*, made out of two round headed brass wood screws and a strip of spring brass as shown in Figure 96, is mounted to the right of the transmitter.
One end of the spring is fastened down firmly with a screw, the other end is bent so that it is directly over but does not touch the second screw mounted about seven-eighths of an inch away.
Four binding posts should be arranged along the bottom of the wooden back and two more just to the left of the transmitter.
The set is now ready to "wire" or connect. The connections can be understood best by following the diagrams in Figures 94 and 97.
Starting with the four binding posts along the bottom of the back, mark the left hand one B1, mark the next one B2, the third E and the fourth L.
Connect a wire from binding post B1 to the post P1 directly above it. Run the wire from P1 to the brass strip on the push button. Connect binding posts B2 and E together and from E run a wire to the right hand binding post on the bell. Connect a wire from the left hand post on the bell to the lower contact 2. The hook should be connected to binding post L. The upper contact 1 is connected to one terminal of the transmitter. The other terminal of the transmitter goes to binding post P2. The telephone receiver should be connected to the posts P1 and P2 by means, of flexible wire. The contact on the push button should be connected to the upper hook contact 1.
The set is now complete, ready to connect to a similar outfit.
The battery is connected to the binding posts marked B1 and B2. E and L are the line posts to which the wires connecting the two sets are led.
The wire connecting the two outfits should not be smaller than No. 18 B. & S. gauge. No. 14 will prove better if the line is a long one.
The wire should be very carefully insulated. Ordinary porcelain knobs will serve as suitable insulators.
If desirable, only one wire need be used, the circuit being completed by connecting the post E on each instrument to the ground or earth. The gas or water pipes will form the most suitable ground connection.
The number of batteries required will depend upon the length of the line. The actual number necessary to ring the bell properly can be best ascertained by experiment. It should be noticed that the battery connections at each station are reversed so that the batteries will be series. The negative of the battery at one station is connected to E and the positive pole of the other battery at the other station to the similar post.
When the stations are not in use the receivers should be left hanging on the hooks. In order to call the other station, lift the receiver off the hook. This throws the transmitter, receiver and battery into the circuit. The resistance of the receiver and transmitter, however, are so great that they will prevent enough current from flowing to ring the bell at the other station. Pressing the push button will short circuit the transmitter and receiver and allow enough current to flow to ring the bell at the other station. The other person has merely to lift up his receiver in order that the two parties may talk back and forth.
When the receivers are on the hooks both bells are in circuit ready for either party to call up.