HOME-MADE MICROPHONE.

Auckland Star, Volume LXII, Issue 38, 14 February 1931, Page 3 (Supplement)

HOME-MADE MICROPHONE.

We are obliged to Elgar Curnow, of Moonee Ponds, for sending us details of a home-made microphone with which the young experimenter with electricity can obtain a great deal of amusement. ThG parts are a block of wood, two carbon rods, a. piece of mica and some carbon granules. The body of the microphone is a block of wood about 3incheS by 3 inches by 2 inches. Near one of the square ends drill two 3-8-inch holes in the position shown in the sketch at "a." (The sketch shows the top view.) These are to take two 3-8-inch carbon rods, whicla may he obtained from old flash-lamp dry cells. Chisel out a shallow recess, shown at "B" in the face of the block between the two rods, about two inches square and 1-8-inch deep. This is to hold

the carbon granules. Bore a small hole through the top of the block, extending through to the recess, and shown at "C." Through this hole the carbon granules are to be poured in. Get a piece of very thin mica, about 2J inches square, and glue it over the recess, being careful that it does not crinkle. This is shown at "D." Yon can now put the two carbon rods in place, and take two wires from the metal caps to two terminals at the back of the microphone. Pour the carbon granules through the little hole at the top until the recess is nearly full. The proper carbon granules are expensive, costing about 15/ an ounce, although you would walit only, say, quarter of an ounce, especially if you do not make the recess too large. But substitutes cap be used with quite good results, among the best being granulated coke, carbon rod (from dry cells) and hardwood cliafcoal. Crush the coke, etc., pass through fairly fine gauze to separate the fine particles you want, and then pass through a still finer gauze (as use., for straining petrol) to separate the dust, which is not wanted. In conjunction with one or two stages Of audio amplification this microphone should give good speaker strength when spoken into. A step-up or telephone transformer must be used if you want to use it with ordinary ear phones, but is not required with low-resistance ear phones.