Fig. 92.

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The sound - board is fastened to the inside of the box by a leather hinge glued along the upper edge; and on a slip of wood, the thickness of rubber - ring, a small spring presses at the lower edge of board, which keeps it close to the screw g.

The sound - hole in mouthpiece is 1 in. in diameter, and the box is made of well - seasoned mahogany, 1/2 or 3/4 in. thick, solidity being essential; the internal diameter of box is about 5 1/2 in., leaving a space of about 1/4 in. all round the board, except at the upper edge, where the leather hinge is glued on.

When the adjusting screw allows the diaphragm to press lightly on front of box, the instrument is in order for speaking, and 8 to 10 in. distance gives first - rate results. The voice should be just as in ordinary conversation; music, such as violin, is beautifully heard through a telephone - receiver. Two or three Leclanche' cells are sufficient for ordinary purposes. The microphone can be used well without induction - coil, and can be fixed to any of the existing arrangements with switch and bell. The angle of microphone box is 10° to 12°.

(9) Fig. 93. - The instrument consists essentially of 2 springs secured to a small base - piece, and each supporting at its upper end a piece of ordinary battery carbon. These 2 pieces of carbon are placed in light contact, and the 2 springs are placed in an electrical circuit in which there is also a receiving telephone of the Bell form. The instrument is represented secured to a small sounding - board. The 2 carbon - supporting springs are fastened to a single base by the binding - posts which receive the battery wires. An adjusting screw passes through one of the springs at or near its centre, and bears against a rubber button projecting from the other spring. This simple device, when placed on a table, indicates in the receiving telephone the slightest touch of the finger on the table or on the instrument. Blowing on it makes in the receiving instrument a deafening roar; drawing a hair or a bit of cotton across the carbon is distinctly audible in the receiving instrument. When the device is placed on a small sounding - board, every sound in the room is received and transmitted.

An ant running across the sounding - board can be plainly heard, and a touch upon the instrument or the table which supports it, which without the microphone would be entirely inaudible, can be distinctly heard in the receiving telephone by aid of the instrument, even though miles intervene. When it is placed on a violin, blowing lightly upon the strings produces aeolian harp tones in the receiver, and a song sung to the violin is rendered in the receiving instrument with an aeolian harp accompaniment. When mounted on a violin or sounding - board, it will transmit articulate speech, uttered in any portion of a room of ordinary size; it will receive and transmit the music of a piano, and even the sounds of turning the sheets of music may be heard. Whistling, flute music, and other sounds are transmitted with their characteristics of volume, pitch, and timbre. This instrument, although so very simple, is capable of doing all that has been done by other instruments of an analogous character, and it will be determined by further experiment whether it will do more.

Although carbon contact points are preferable, they are not absolutely essential to the operation of the instrument, as metallic points will do the same things, but not so satisfactorily. (G M. Hopkins.)

Fig. 93.

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(10) Microphone for reproducing Speech (Fig. 94). - It consists of a box of thin wood, the front of which is perforated with a hole large enough to receive the tube of a common string telephone, the parchment membrane d, stretched over the inner end of which, is kept level with the surface of the board on the side at which the microphone is placed. The membrane d carries in its centre a small piece of metallized pine charcoal c7 which is connected by the wire g and binding - screw i to the battery wire. A vertical lever, delicately pivoted on 2 points at h, carries at its upper end another piece of similar charcoal p, which is lightly pressed against the piece c. The lever is connected with the circuit by means of the wire and binding - screw j, and the pressure with which it bears on the charcoal, carried by the membrane d, is regulated by a light spring and silk thread actuated by the tension screw e. With a battery of 6 or 7 Leclanche' cells, words can he transmitted and received; but they are always much less accentuated than with the Bell telephone.

The apparatus, however, appears to be a neat and handy form of microphone to employ for speaking purposes, and can be made very cheaply. (Th. du Moncel.)

(11) Hughes's Microphone

In order to hear the tramp of a fly, the microphone is constructed as in fig. 95; a stick of carbon (preferably the round compressed pencils used for electric light), pivoted on its brass sopport at c, and resting by the slight pressure of its weight upon a small block of metallised charcoal (blow - pipe, box - wood, or any hard conducting charcoal will serve); the wires are connected to the pivot support at c, and charcoal 4. This structure is fastened to any small board J of pine, 1/6 in. thick, and 3 in. or 4 in. square. This will also perfectly transmit articulate speech, if spoken to not closer than 1 ft.; if louder tones are desired, put a small weight on a, and speak within a few inches of the microphone.

Fig. 94.

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Fig. 95.

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(12) Making Plates For Microphones

The following process for making very thin plates for microphonic purposes, is given by Trichasson, of Hour - melon - le - Graod. A sheet of ordinary tinned iron of any thickness is cut into plates, and rubbed vigorously on both faces with a dry linen cloth. This operation is to clean away all grease. A plate is then taken and plunged vertically in a bath of nitric acid until it is entirely immersed. Acid which has already served in Bensen batteries will answer very well, or better still, the nitric acid of commerce, diluted with 1/4 water. It is necessary to remove the plates from the bath from time to time, in order to see whether the required thickness has been reached. When this is attained, the piste is washed several times in water to remove the black layer of oxide. The plate is then allowed to dry, and afterwards varnished on both sides with Japan varnish, to prevent oxidation. This process permits of making microphone and telephone diaphragms as thin as may be desired, and at little cost.