This microphone, with multiple contacts, as shown in the accompanying figure, is composed of a mouthpiece, E, affixed to the end of a glass tube, T, one centimeter in diameter, itself fixed on a jointed stand, thus enabling the whole apparatus to be moved at any inclination.
The mouthpiece contains an ebony plate one millimeter thick, on which is fixed a piece of copper, M, penetrating the glass tube a slight distance. In this tube there are six carbon balls slightly smaller in diameter, so that they can easily be moved.
The microphone is completed by a second piece of copper, M2, supported on the end of a hollow breach, K(R), by means of a little spiral spring, not shown in the figure. The screw, V, fixed in the cup, Q, serves to regulate the pressure of the piece, M2, against the balls. The variations in the resistance of the microphone are reproduced equally through all the contacts of the balls, because, when talking at the mouth-piece, the vibrations are transmitted almost instantaneously, as in the well known case of billiard balls.
The apparatus acts like an ordinary middle sized Gaiffe microphone, with six elements (peroxide of manganese and chloride of zinc) set up with a resistance of 800 ohms, with a Bell telephone for a receiver.
By employing inductive currents and a fine wire telephone receiver a necessity with inductive currents—the distance may be largely increased, and extended, with artificial resistances, to 250,000 ohms.
We have been present at experiments made with this microphone, and we have found that it transmits the voice very clearly, without altering the tone and without any scratching sounds.—Electrician.
February 5, 1881 Scientific American