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
Fig. 5.—Sectional view of 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.