A Home-made Microphone.—Some of the readers of this journal may be pleased to have a description of a little microphone that has given good results, and which can be made, in a few minutes, from material at hand. It is represented in the figure of the natural size. It is made from a visiting-card of the ordinary thickness cut square. A round card might look better, but it will give less satisfaction. On the card should be fastened with sealing-wax three thin, light disks of carbon, BBB', of the quality used in the electric light.
The disks should be placed symmetrically at the angles of an equilateral triangle, and should be put in communication with each other by the copper wires bbb’ which are either soldered or stuck tightly into holes made in each disk to receive them. Platinum may be advantageously substituted for copper. The rest of the apparatus consists of a square wooden foot, M, supporting three prismatic rods of carbon, CCC', arranged so as to correspond exactly with the three disks BBB'. Two of the rods, CC, communicate by the copper or platinum wires dd with the common terminal D, while the third rod, C, communicates alone with a second terminal, D'. The upper ends of the charcoal rods should be cut into a bevel-shape—not into a point, for that does not give sufficient contacts. The rods are sealed to the wooden base M. The theory of this microphone is very simple. The current enters, for example, by the terminal D', follows the rod C, then the disk B', whence by the wire b it passes by the two disks B to return to the terminal D through the two rods CC. The little instrument may be made very sensitive to the voice and to all sounds, provided the card A is given the proper weight, and is neither too heavy nor too light. The voice, with its timbre, of a person speaking in his usual tone at the other side of the room, can be heard very distinctly in it. The sounds of the piano are particularly well reflected. The apparatus should be placed upon a table two or three metres away from the sound. For a battery to put the microphone in action, I have generally used a small Bunsen element. Two or three Leclanché elements would do as well. I have used a modification of the Leclanché elements, in the shape of a pile made of a plate of zinc and a carbon plate, moistened with a saturated solution of bichromate of potash and hydrochlorate or sulphate of ammonia. It is in fact the bichromate pile without the costly mechanism which is used for relieving the zinc from the action of the acid when the apparatus is at rest. This element does not waste when the current is interrupted, as in the Leclanché pile. A difficulty which arises in the use of the pile, from the penetration of the carbons by the ammoniacal solutions till they attack the wires, has been obviated by M. Préaubert's device of exposing the carbons to a bath of
