Telephone Construction Part 4 300122

(c) Fig. 118, making the bell - case act as switch - board, 4b are the line ter - O 2 ininals; a is in connection with one terminal of bell, battery, and telephone respectively; b is connected with switch arm f, moving over studs 1, 2, 3, which are the other terminal! of bell, buttery, and telephone. Dotted lines on the box show the connections inside, which can be easily made out if each circuit be followed separately. The action has been most satisfactory.

Fig. 116

Telephone Construction Part 4 300123

(d) Fig. 119: it is a brass wheel, the edge of which is milled, and which runs on the axle r connected with the wire rs The end of the wire s is connected with one of the poles of an electric battery, the other pole of which is connected with the earth - plate or return wire; ed and ab are springs, a being connected with the earth - plate through the telephone, and a with the line wire; crd is a Sat piece of ebonite (or wood steeped in paraffin), with two brass studs d c fixed into each end, and in metallic connection with one another by a wire this ebonite is firmly fixed on the wheel, so as to insulate the studs d c from it. The dotted circle at the bottom of the wheel represents a lump of lend, fixed behind, so as to keep the wheel In the above position when not being turned by a handle fastened in front. It will be seen that when the wheel is turned out of the above position, the spring ed will press upon the milled edge of the wheel, and thus establish electrical communication between the wire e and the wire a, which is connected with a battery.

But when the stud at d is moved round, the stud at c will also move, and the spring ab will then be released, and tend to press on the edge of the wheel, from doing which it is restrained by a small pin in front of the spring at b: In the position shown in the figure, a current entering at e from the line wire would take the course edoa, and then go back through the telephone and earth. When the wheel is turned, however, any connection between the spring ab and the wheel is impossible, on account of the pin at 6, but the spring ed presses on the milled edge of the wheel, which is connected with the battery, and thus, while the wheel is being turned, numerous small currents are sent along the line, which generate a load musical note in the telephone at the other end, and thus call attention. No hells are used, and when attention is to be called at the other end of the line, a handle has merely to be turned; the trouble is thus extremely slight. (T. A. Garrett.).

Fig. 119

Telephone Construction Part 4 300124

(6) Augmenting Sound (Fig. 120). - A simple contrivance for augmenting the sound of the telephone, by increasing the variations of current by pressure on a tube filled with powdered carbon, caused by an electro - magnet. The cur- rent from the battery b flows through carbon telephone c, where it meets with varying resistance; from this it goes to the electro - magnet m, which it magnetizes and causes to attract a small piece of soft iron attached to the spring s, and produces pressure in the glass tube t, which is tilled with powdered carbon. From the magnet the current passes to a wire introduced into the lower part of t, the bottom of which is stopped with a cork; from this wire it passes through the powdered carbon to another wire higher up in the tube. When the current is strongest, there will be the greatest pressure on the carbon, and so the least resistance; when it is weakest, there will be least resistance. The tube by itself is shown on the right. The bottom is stopped by a piece of cork k, through which are introduced the wires p and n, of which p is covered with an insulating substance as far as the bent part.

In the top of the tube is fitted a piece of thin iron, against which the spring is caused to press.

Fig. 120.

Telephone Construction Part 4 300125

(7) Cheap Magnets

Procure from a saw - sharpener 2 old 6 - in. 3 - square files, worth not mors than Id. each. Put one in a vice and knock the shank end off (that which fit. in the handle) to within 1/4 In. of main part of file; take out, measure off 5 in. from shank, put the file in the vice the reverse of what it was, and knock the other end off, as near as you can so that it will be 5 in. In total length, and you will have (when magnetized) a magnet with an iron pole - piece, the shank being much softer than any other part of the file. Grind the shank round as near as you can, for your coils to go on; with such magnets you can lift a 1/4 lb iron weight. They also work through greater resistances than the roll steel commonly used. They apeak very plainly, Magnetize them with, S horse - shoe magnets, putting 2 on the table with their opposite poles 4 in. apart. Bridge the spaces with 2 files, and use the third for magnetizing. Commencing at the centre of the file, rub it to one end of the file, then return to the centre and pass to the other end, and so on, finishing at the centre, and slide the magnet off; then proceed with the other one.

(8) Circuits

(a) Switches for telephone, microphone, and bell (Fig. 121).

Fig - 121.

Telephone Construction Part 4 300126

To ring bell, move the brass slip or. working on a, to form ad; and upon moving bd, working on b, to form be, the bell will ring if your battery is strong enough; if not, place a piece of copper wire to join your telephone terminals, which will take out the r of the coil within, as shown. The receiver hearing the bell, turns vy, working on v, to form vx, and then wx, working on to, to form wy, and answers. The transmitter now may speak by the microphone, or by replacing strips as shown by telephone alone, df is to cut out the telephone and use microphone alone. The message ended, all strips are replaced as shown; the battery circuit is then broken until you require to " telephone " again, proceeding as before. Telephone wire is usually small; consequently, the resistance through it, the telephone coil, and coil bell, is considerable. (Hannen.)