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.
Fig. 6.—Connection of microphone and telephone.
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.
Fig. 7.—Experiment with microphone.