Take the carbon pea which you have roughed down in the lathe, put it in the hole in the steel plate, rub it round in every direction between the finger and thumb, and after a while it will pass through the hole and be almost a true sphere. I have made the carbon bails by both these processes, and though the latte. is by far the more accurate, the former has answered very well. One plate will make a great many balls. In all microphones, the points actually in contact, or which regulate the current, oxidize, and when this is complete, the current will not pass. In this cup - and - ball microphone, fresh surfaces are constantly coming into contact, and a shake will always ensure this. If the microphone is properly constructed, the ball should rattle loudly - when the microphone is shaken. In screwing up the microphone, the diaphragm should not be pinched too tightly. The best way is to screw it up a little too tight, and then slacken it slightly. This microphone need not be round in shape; it can easily be made square, in which case the dimensions should not be less than 4 in. by 3 in., and a mouthpiece may be dispensed with.

I have made a microphone with 4 sets of cups and balls, the diaphragm being about 7 in. in diameter, and the cups arranged with 3 pairs equidistant in a circle 3 in. in diameter, and the fourth pair in the centre; the cups in the diaphragm being connected together, and the cups on the back or case also connected; with this arrangement, a very strong battery may be used, when the articulation is a little louder, but carcely so distinct as with one pair of cups only. If a multiple microphone is made up, the cups should not be leas than 1 1/2 in. nor more than 2 in. apart.

The receiver is made as follows: - Take a round piece of mahogany, or other dry wood, 2 in. in diameter and 1/2 in. thick; in the centre make a circular hole the size of a sixpence to take the electro - magnet: on one face mark a circle the size of a bronze penny; just outside this circle, and touching it, make 3 holes equidistant from one another, and the size of No. 9 B.W.G. iron wire; in each hole put a piece of this iron wire, long enough to project a little from the wood on each side; the electro - magnet is made of a piece of the same No. 9 B.W,G. iron wire, slightly less than5/8 in. long, and the reel is the size of a sixpence, 1/2 in. long, and wound with No. 36 silk - covered copper wire; the electromagnet is placed in the hole in the wood, the end of the wire is carried out to connect with the terminals, and. the whole is boiled in paraffin. Two pieces of thin wood, about 3/15 in. thick and. 2 in. in diameter, are also required. In one a recess is made the size of a penny and rather less than 1/16 in. deep; in the other a similar recess, but with a central aperture about 1/2 in. - in diameter, to permit the sound to escape; 2 discs of thin iron, 5 mil. thick, or ferrotype ..plate, are also required.

Take, the disc of wood containing the electro - magnet, the wires of which have of course been led to convenient terminals, file down the ends of the 3 iron wires on one side of the disc, until one of the thin iron discs, when laid upon these 3 iron wires, will almost touch the core of the magnet; let this plate remain on these 3 wires, put on the recessed disc of wood, which is without the central aperture, and secure it by 3 screws. The setting of the other thin iron plate requires more care, hut is done in the same way, the projecting ends of the thin wires being filed down until the other thin iron plates, when placed upon them, and the whole put in circuit with an articulating microphone, speaks distinctly: the recessed disc of wood with the central aperture is then placed upon the iron plate, and secured by 3 screws, which must not be tightened too much, or the plate will not speak clearly. We hare now an electro - magnet between 2 thin iron plates, which plates are in metallic connection with one another by the 3 iron wires; one plate being adjusted at the best speaking distance from the core of the magnet, and the other plate as near to the core of the magnet as it can go without touching it at any time.

The above directions will enable any one to make both the cup - and - ball microphone and the electro - magnet receivers; but remember that a good receiver is of no use with a bad transmitter, nor a good transmitter with a bad receiver. I have had these instruments in use for 2 years: they work admirably and give no trouble. For telephone work, I prefer the gravity Daniell battery to any other, but I intend to try the iron perchloride battery. (H. B. T. Strangways.)

(8) Fig. 92. • - The diaphragm is constructed of white or yellow pine, 1/12 in. thick and 5 in. square; on the edges are glued 2 strips of wood a, about 3/16 in. thick, shaped like a bridge, about 1/2 in. at the middle, and tapering gradually at the ends; these are glued across the grain, and prevent the soundboard b from twisting. The carbon pencils c are made from electric - light pencils No. 2, obtained from E. Pater - son, 76 Little Britain, and the pencil - holders from No. 5 pencil. The pencils are 2 in. long, neatly filed to a point, and fitted into a small leaden tube d. The tube used for pneumatic bells answers well. Round the carbon is glued a leather collar e, which secures the lead in place, and acts as a damper in preventing the sound given by the carbon itself. This is heard if a small piece of carbon is struck, and is the cause of the metallic noise so often heard in microphones. The length of the leaden tube should be 1 3/16 in.

The pencil - holders are cut about 7/16 in. long, a hole is drilled half - way through, and a groove is cut round, as at f; 4 holders are glued to the sound - board, and, when dry, the pencils are put in place loosely by placing the remaining 4 holders. Connections of copper wire, cleaned well, about No. 30, are placed in the grooves in the carbon holders, twisted up, and a touch with soldering - iron afterwards makes all secure. On the opposite side of diaphragm is lightly glued a rubber - ring, about 2 1/2 in. in diameter, which rests when in box against the front, and the pressure is regulated by a screw g, filed to a square, and moved by a key. This rubber - ring acts as a damper, and prevents noise and rattling.