CHAPTER XLIX

HOW TO MAKE A TELEPHONE

FOR USE AT HOME

IT is quite easy to make a telephone, which besides affording amusement, will, at times, be found extremely useful. Fitted from the ground floor to the rooms above, or from your friend’s house to your own, it is impossible to overestimate its convenience.

Of course it is quite possible to buy the parts of a telephone ready made and fitted together, but it is much more interesting to construct the apparatus yourself.

The whole affair is so easily and cheaply made that no difficulty can possibly be experienced, and you have only to exercise care in order to succeed in constructing a very handy and interesting machine.

For the apparatus to be of any practical use, a pair of receivers are required--one for the person speaking, and the other for the person listening.

First of all purchase two bar-magnets, _i.e._ straight and magnetized at one end only--not the ordinary horse-shoe magnets. These bar-magnets should be about four inches long, and can be bought from any electrician; at the same time obtain from him an ounce of silk-covered wire, No. 36.

Before taking the magnets from the shop it would be as well to test them. This can be done by placing the marked end of one to the unmarked end of the other, and unless the former is able to support the weight of the latter, do not take either, but insist upon the electrician giving you stronger ones.

Having made your purchase, set to work upon the actual apparatus. The first things to be made are the wooden cases for the magnets, and these require care in construction.

From a straight broom-handle cut off two lengths, each a little longer than the magnets, _i.e._ about 4¹⁄₂ inches long, and split them carefully down the middle with a chisel. Along the center of each of these halves cut a groove (B, Fig. 1_a_), in such a manner that when the two halves are put together they fit properly, and there is room for the magnet to slip backwards and forwards in the hole running through them. Now glue the parts together and you will have two short wooden tubes, or magnet cases, as shown in Fig. 1_b_.

MAKING MOUTHPIECES

Having made the cases for the magnets, now make the mouthpieces. Procure two wooden boxes such as boot-buttons are sold in, and glue one to the end of each of the wooden tubes you have just made, cutting a hole in the bottom of the box to correspond with the bore of the case. In Fig. 1_a_ you will see the work, so far as you have completed it, shown in section. In the lid of the box cut a clean hole the size of a dime right in the center, but this lid must not be fastened to the box just yet.

The next things to be made are two bobbins upon which to wind the wire--one for each instrument. Cut a piece of fairly strong cardboard about ¹⁄₄-inch wide and just large enough in circumference to fit the marked end of the magnets tightly. Divide your No. 36 wire into two equal lengths, and wind each piece around the bobbins, leaving the two ends free to the length of about two inches.

Next slip the magnet, M, into the groove, and push it through so that the bobbin, L, is contained in the box C, Fig. 2. Make two holes in the bottom of this box, pass the ends of the wires through them and fasten them with binding screws, as in the illustration H, Fig. 2.

The thin plate which vibrates with the voice must next be made. Obtain a smooth piece of very thin sheet-iron--of just such a texture as is used by photographers for ferrotypes. Take the greatest care to keep this perfectly flat. Cut two round pieces large enough to cover the mouthpieces as you will see in P, Fig. 3, where it is shown in section. Take the lid in which you have already cut the circular hole and fasten it to the box in such a way that it keeps the iron plate in position (Q, Fig. 3).

Push the magnet very close to, but not quite touching, the disc of iron--say ¹⁄₁₆th of an inch away, and fasten it in that position, either by means of a wooden plug at the other end, or by driving a screw into the case in such a way that it grips the magnet and holds it firm (S, Fig. 3). Fill up the hole at the other end of the case with a piece of cork, glue or screw on the lid, keeping the opening so that it almost touches the iron disc, and your telephone is complete. Fig. 3 shows in section how it should now appear.

It remains only to make the connections with the batteries in the manner shown in Fig. 4, and you will be able to speak with your telephone. It is very possible, however, that the voice will carry faintly, but do not be disappointed, as this can be very easily remedied by making an apparatus which is usually attached to the telephone, and is known as

THE MICROPHONE

This instrument, as its name implies, serves to make small sounds greater. Besides being useful in conjunction with a telephone, it is also of interest in itself.

Make a base, BC, and an upright, A, from a piece of thin pine wood, as shown in Fig. 5. Buy from the electrician two blocks of carbon such as are used in batteries (these will cost but a few cents), each measuring about a cubic inch. In one side of each of these blocks fasten a long binding screw, whilst in an adjacent side make a hollow with a rat-tailed file or a penknife. Then get a small pencil of charcoal such as is used in the large arc lamp, and cut off a piece 4 inches long; file both ends to a point in the way that you sharpen a pencil (E, Fig. 5).

Having made these various parts, fit them together. Screw the two pieces of carbon to the upright in such a way that one rests upon the base whilst the other is at such a height that the charcoal pencil rests lightly in the hollows, between the two carbons, as in DD, Fig. 5.

The microphone is now complete, and can be attached to the telephone as shown in Fig. 6. It will be found to increase the sound to such an extent that your voice will be distinctly audible, and conversation perfectly clear.

By attaching one receiver and the battery to the microphone as shown in Fig. 7, very slight sounds will be increased to an extraordinary degree. The ticking of a watch seems like so many heavy footfalls, and if you can succeed in getting a fly to walk across the sounding-board, as the base is called, the footsteps will be plainly heard. Many interesting experiments can be performed in this manner, and as the microphone is so easily made, it is well worth constructing one for the sake of such experiments alone.