As far as Blyth's experiments go, the following appears to be something like the true explanation of the microphone action. What he has termed the air and the tremor effects take place simultaneously. The tremor effect produces a jolting of the carbons sufficient to allow momentary minute electric arcs to take place between the points which are just clear of contact with each other. Simultaneously with this, the air effect comes in, and on account of the variations of density due to the condensations and rarefactions of the air, acts upon the minute electric arcs so as to vary their resistance. The tremor effect explains merely production of the musical pitch of the sounds heard in the telephone, whereas it is to the air effect that we must look for the transmission of the quality of the sounds uttered into the microphone transmitter. The microphone is thus so far a delicate make and break analogous to the old Reiss transmitter, with the important addition, however, of minute momentary gaps filled with a material which is sensitive to the minute harmonic variations of the atmospheric density which constitute sonorous vibrations. (Prof. Blyth.)
(1) Simple Microphone, capable of making the tramping of a fly, etc, audible (Fig. 86). - All the battery it requires is a piece of carbon and zinc or copper and zinc, about 3 in. square, with a piece of blotting - paper between, damped with vinegar. It answers equally as well as an expensive battery. / is a box, size immaterial, say 4 in. square; over the top is stretched a piece of vegetable parchment; a, pieces of carbon filed to a knife - edge at top, to support small stick of carbon, b; c, piece of wood glued to a and to parchment top; d, wires connecting a with binding - screws e. An old sewing machine needle makes a good drill for drilling the holes into which the wires are wedged tight with the point of a pin. B is another form, scarcely so sensitive, but less liable to accident from flies walking over it. a are 2 pieces of carbon about 1/4 in. square, fastened together by being glued to a thin piece of card which reaches about half - way up. The top of the carbon, in the centre, is - cupped so as to hold a small pellet of carbon, ranging in size from a mustard - seed to a pea; b, wires to binding - screws. When all the connections are made, place a watch at the bottom of the box at g, and gently push the carbon piece b till it is almost falling over.
A little practice will soon enable any one to get a very fine balance. The finer the balance, the better the result. When the ticking of the watch can be heard distinctly, take it out and place it on the top of box, resting the ring on the piece of wood c. With a moderately good telephone, the ticking will be sufficiently loud to be heard across a good - sized room. For flies, cover the top with a bell glass, or put them in the box and close up the opening. When using a common pin instead of b, flies may be heard running about almost as distinctly as with the carbon. A human hair drawn across the - parchment is heard as a rustling sound in the telephone. The experiments should be conducted in a quiet room, as the slightest conversation or movement affects the microphone, and produces a jarring noise in the telephone. (T. Cuttriss).
(2) Adjustable Microphone
Get a piece of 1/8 - in. deal, about 4 in. by 3 in., polish it up, and ebonize it. Underneath put 4 corner pieces, 2 of which are shown at A, Fig. 87. Next fix one of the carbon pieces on the board. In one corner of the board cut a square hole through, as at a, then get your carbon block and bore a hole half - way through it, making the hole wider inside (A). The carbon "block is sunk slightly into the hole at a, a piece of copper wire is inserted, and melted lead is poured into the hole, binding the carbon block on and making a good connection between it and the wire, which is then connected underneath to the binding screw at 6. Then get a sb of thin " latten " brass about 2 in. long b. 3/16 in. broad, and punch two small holes in it, one at one end c, the other in the middle d. At the other end a piece of gas carbon, about 2/3in. by J in. by 1/8 in., is fastened by means of melted lead The end c is then fastened down with a screw and washer, the carbon end in this position being about J in. from the carbon block. A small spiral is made with No. 32 copper wire - this is put on a taper wood screw, which is screwed through d till the 2 carbon surfaces touch. If small sounds are intended to be shut out, the screw is tightened.
It may, of course, be used with a soundboard if desired. (Eng. Mech.}.
(3) Gives excellent results in transmitting the human voice and musical notes (Fig. 88). The upright board has a circular hole a cut through it, about 2 1/2 in. in diameter. Over this is gummed some vegetable parchment b, which, when the gum has well dried, is wetted and dried several times till quite taut. To the centre of this is fixed the upper carbon block c by means of a screw and a small wooden nut, and the wires are connected with battery and telephone in usual manner. I have only tried a 2 1/2 in. hole, but am inclined to think that an improvement may be made by varying the size of the hole and the position of the carbon block on it. (T. J. Mercer.)
(4) Will transmit distinctly the loudest voice and the lowest whisper, when such are spoken 10 ft. or 12 ft. distant, without the smallest jar, and in the same tone as the speaker's voice. (Fig. 89.) Take a piece of very thin deal, 5 in. by 2 1/2 in., smoothly planed; fix to it the sides 1 1/2 in. deep, equally thin; now add ends 1/8 in. thick. You will then have a lidless box whose bottom and sides are very thin and smooth, but with ends much thicker. Hold a stick of cask - wax in the flame of a spirit lamp, and run it in the seams where the sides and ends join - of course having previously glued them; screw this firmly through its end, to a stout base - board 3 in. by 7 in. In this box fix an ordinary microphone; to the centre of the vibrator cement a piece of iron wire. It is only necessary now to make a stand upon which to place a horse - shoe magnet; the stand, with the magnet upon it, must be in height so that when placed upon the base - board the feet of the magnet will stand parallel to the iron wire. The magnet may be fixed to the stand, but the stand must be free, so that it can be moved backwards or forwards on the base - board, nearer to or farther from the vibrator.