To call the distant station, press the button a, which breaks contact at b, and puts the line / and carbon of battery c in contact at d. The circuit is completed by putting the zinc of battery to z, whence the current passes through the receiver m, and out at e either to earth or return wire. By this arrangement, the ringing at the distant station is repeated or made known at the sending station. The contacts at b are made with two strips of hard rolled metal, and continued under the lever gt which must be kept down on f while signalling with the bells, but be in contact with i when speaking. This is done by hanging the ear - tube on the lever hook, or the Bell receiver, should that be used instead of the electromagnetic one. In coupling the instrument, the / of one is connected with the e of the other. The transmitter swings on 2 small brackets inside the' front, and contact is made at J and k when the front is closed. The carbon - cups are made out of 1/2 - in. carbon, and are best done with a rose countersink, large enough to take half the ball without touching the head of the screw which secures the cup in its place, or the cup may be glued in its place, contact being made by twisting a wire round the outside; the discs are made from slices of carbon - pencil with a groove round them to have wire twisted, and when the balls are screwed up in the case they should be quite free to rattle.
The cups should be arranged 1 in. apart. An old saw and a file will do all the cutting and shaping, and if you have no lathe, a brace will answer the purpose. In the receiver, a bundle of iron wires takes the place of the permanent magnet, which was devised to meet the case of "Bell's "disclaimer, in which he disclaims a magnet excited by a battery in the line circuit. The adjustment of the diaphragm has to be made before finally screwing up, and will give no trouble if you observe to file the faces of the body of receiver and the core a dead leveL Then cut a ring of stout writing - paper, with the hole about 2 in.; glue one on each side, and when the ferrotype plates are laid on, they will be the right distance from magnet, and may be screwed up. Instruments switched in this man-ner are as loud as any made; but they can only be used in pairs, unless a separate switch is used with each instrument to reverse the poles of the battery. But in order to remove that obstacle, if the receiver is formed of a small induction - coil instead of the simple one, then an alternating induced current is produced, without using an induction - coil in the instrument.
The result of this will be that any resistance in long lines may be overcome by simply increasing the battery power to increase the power of the magnet, the induced current being proportionately strengthened.
Fig. 126 represents the 2 boards, one at each end of line, fitted with bell a, bell - push d, telephone, telephone - hook /, and terminals for transmitter b; also 4 terminals on the top of each for carbon of battery c, zinc of ditto z, earth e, and for line. Note the difference in battery wires in No. I. and No. II. stations. The push d is a 3 - line push, line and bell being in contact; when pressed, bell is cut out, and line and battery are in contact. The hook is balanced on pivot /. When telephone is on it, the other end is in connection with bell stud; when removed, it falls to the other stud, and puts bell out and telephones in circuit, The currents pass as follows: - No. I. to ring up No. II.; c to push d and line, enters No. II. by line to push, hook, bell, earth, to e No. I. to z; No. II. to ring back; c to e, to e No. I., to bell, hook, push, line, enters line No. II. to push, z and c. To speak No. I. to II., unhang telephones each end; c to transmitter, telephone, hook, push, line; enter line No. II. push, hook, telephone, transmitter, z through battery to c, e to e No. I. and z. To reply No. II. to No. I. c to e, to e No. I., z, c, transmitter, telephone, hook, push, line.
Enters No. II. line, push, hook, telephone, transmitter, and z.
The following works will be found exceedingly useful to those interested in the subjects of which they treat: - A. Bromley Holmes, 'Practical Electric Lighting;' R. E. Crompton, 'The Electric Light for Industrial Uses;' T. B. Grierson, 'Electric Lighting by Water Power;' Killingworth Hedges, 'Useful Information on Electric Lighting;' T. D. Lockwood, 'Practical Information for Telephonists;' and the article on 'Electrical Engineering' in the Supplement to Spoils' Dictionary.