A microphone is one of the most important and interesting electtrical instruments which the amateur who is interested in electricity can construct. And, although extremely sensitive in its action, it is very simple in construction. For these reasonsthe one here described will be found of interest to those who are studying telephone work, as from it the principles governing the transmission of minute and varying currents can be readily studied.

The materials required include several short pieces of arc-light carbon, a cigar-box, two screw-eyes, a small three-cell battery such as used in night lamps, and a telephone receiver. The latter can also be made by the reader as described in the December, 1902, number of this magazine.

The illustration shows the general construction, A being a base board for holding the cigar-box upright, C and C' being pieces of arc-light carbon about 4 in.

long. A piece of wood W, 4 in. long and 1/2 in. square is attached near the top with the two screw-eyes E, first boring holes 3/4 in. from each end so that the screw-eyes will slide freely therein. Two small springs 1/2 in. long are made of fine brass wire, which may be easily done by winding the wire around a wire nail of a little larger size than the. serew-eyes ; the latter must be, therefore, about 1 1/2 in. long in the straight part, but may be of small wire gauge.

The piece of carbon C' is first attached to the box by winding two or three turns of bare copper wire P tightly around each end and then carrying the ends through holes bored in the box, and there joined together by twisting several turns close to the box and then connecting to the inside end of the screw or binding post S.

The piece of wood, W, also has a vertical hole bored in the center to receive a short piece of flexible telephone wire, D, the lower end of the latter being fitted into a small hole drilled in the end of the other piece of carbon C, and held firmly in place by a wooden plug or by soldering. It will be seen that by turning the screw-eyes in or out the pressure of the carbon C on the piece C can be nicely adjusted. The inner end of the wire D is carried to one terminal of the battery, which is simply placed inside the cigar box. The other terminal of the battery is connected to the inner end of the screw S'. The telephone receiver R is then connected by flexible wires to the outside ends of the screws S and S', small washers having been placed under the heads to give greater and firmer contact.

In operation the microphone should be placed on a firm, level surface and the piece W adjusted by turning the screw-eyes E so that the carbon C rests very lightly on the piece C". By placing the receiver to the ear the faintest sounds will be transmitted to the ear. A fly walking on the box will cause a quite audible sound, and the ticking of a watch on the top of the box will seem like a tremendous racket. By carrying wires connecting the receiver to another room, considerable amusement may be derived by having visitors guess the cause of the sounds that are heard in the receiver and then seeing their astonishment upon being told the real cause.