Fig. 117 is a microphone which any person who has the construct for himself. The vibrating plate A consists simply of a visiting-card of medium thickness cut much better than round, as the latter, although more elegant in appearance, docs not give so good results. To this card are affiled by means of sealing-wax 3 thin and light discs of carbon BBBof the kind used for the electric light. These 3 discs occupy symmetrically the 3 apices of an equilateral triangle, and are put into communication by means of copper wires b. With this object in view, a small aperture ia formed in each disc, and into this is fixed the extremity of a copper wire either by cement or friction. The copper may be advantageously replaced by platinum. Finally, the 3 wires are
The rest of the apparatus consists of a square wooden base M, which supports 3 prismatic carbon rods CCC, that exactly correspond to the 3 discs BBB. The two rods CC communicate by copper or platinum wires dd with the same terminal D. The third rod C communicates alone with a second terminal D, The upper extremity of these carbon rods must be chisel-shaped, such a form having been found to give the beet results, inasmuch as the contacts become fewer in this case. The rods are fixed to the wooden base by means of sealing-
The theory of this microphone is very simple. The current enters, for example, through the terminal D, follows the rod C and then the disc B. From the latter it passes through the wire 5, into the disc BB, to return to the terminal D, in traversing the two rods CC.
This little instrument will prove very sensitive to the voice and all noises, provided that the plate A he given a Eroper weight, one that is neither too heavy nor too light. If this be done, the voice of a person speaking in an ordinary tone may be distinctly heard at the end of the room that contains the particularly well rendered by it The apparatus must be placed upon a table at a distance of 2 - 3 yd. in order to protect It from the jarring of the earth.
As Tor the pile necessary for saturating the instrument, one small Bnnaen. or two or three Leclsnche elements may be used. Apropos of the Leclnnche" pile, a modification of it formed of a zinc and a carbon plate, both of them dipping into a saturated solution of bichromate of potash and hydrochlorate or sulphate of ammonia, is very simple, and avoids the costly mechanism designed to remove the zinc from the action of the acid when the pile is at rest. This element does not wear away when the current is interrupted, as in the Leclanche' pile. One obstacle at first rendered the use of this pile very difficult, and that was the fact that the ammoniacal salts rose along the carbons and attacked the communicating wires so that these broke and thus interrupted the electric current. But it is only necessary to dip the carbons into a bath of boiling paraffin, then allow the whole to cool, and afterward to scrape the carbon with a knife so as to free its surface of the paraffin. This latter material penetrates the pores of the carbon without notably changing its electric conductivity. The liquids are thus no longer able to rise through capillary attraction. Leclanche got around the difficulty by leaden armatures, but the means described above are simpler.
The electromotive force of this new element appears to be greater than that of a Leclanche of the same dimensions. (La Natture).
Easily constracted microphone.