(B) Telephone Circuits And Call (Fig, 122)

When the button r is pressed, the spring h makes contact with support s.

A current then flows from battery to c through spring A by means of a wire to b, thence through A to the line. Through the line it enters the other end through kbh to d; from d through coil / to terminal t, through the keep and frame g to earth, ringing the bell n as it does so. On taking the telephones off the hooks at both ends, the currents produced by speaking into them are sent through a to A, and then to line entering A at the other end through a to the telephone there, and from end 2 of bobbin to earth or return wire. These connections can be made out of any old pieces of brass, screws, etc, and fitted up in a box. So long as the contacts are clean and firm, it will work as well as if better made. A, automatic switch arrangement, brass hook pivoted so that a spring fastened to it and point p would keep it down firm on a if telephone m were taken off; a is connected with one end of telephone bobbin; 6, brass post so placed that A presses firmly against it when m is on the hook; A, brass.spring normally pressing firmly upon d, and capable of being pushed hard against c by means of ebony button r - it is connected by a wire to b; c, brass connected with positive pole of battery; d, brass post connected to one end of coil of bell movement /; 2, an end of telephone bobbin connected to earth or return wire.

Fig. 123.

(c) Ditto (Fig. 123). - This is the same as supplied with the Blake's Transmitter, and is as simple as any. The hook e has the telephone off; the spring / has drawn the hook contact down on contact 2; s is the secondary wire of the coil; a current coming from s passes through contact 2 into hook to b, through contact to a to terminal /, this being line. The other end of secondary s is fixed to terminal t; f is connected to terminal ze; zinc of battery and earth is connected to terminal ze - hence the two letters; the telephone cord is - connected to tf; the primary wire of coil, one end p to microphone, the other end g to contact 1; A is terminal of transmitter zinc; c is terminal for copper or carbon end of battery. When the telephone is on the hook, it draws it down and places hook contact in contact with bell contact 3. A current now being sent from the other end to ring up, passes through lab to 3 through bell coils, regulating screw 4, through keeper and frame to ze. The act of taking off the telephone allows the hook to slide off the vulcanite v, and lets the ring 5 come in contact with contact 1; this puts c in connection with one end of primary p; the other end g was already in connection with h through the microphone. The instrument is now ready for speaking to.

A current coming through line passes through ab, hook 2, secondary wire to 6, through telephone cords, coil, f to ze. To ring up, a is pressed on d; this does not affect your own instrument, as a leaves b before it touches d, so cut your own instrument out. (Tolman.)

Fig. 123.

(d) Transmitter and Switch (Fig. 124). - The pantelephone is a good transmitter when combined with an induction - coil, not without. The hanging plate may be made of pasteboard, also of thin wood, with like results, but the best effects are got when the contacts are of carbon only. The figure shows a very useful switch suitable for working the 4 instruments required in successful telephony - viz., the telephone, microphone, bell, and battery. To call attention at the distant station, where it is assumed the same kind of instruments are in use, press the button 6 on switch for a second or two; this breaks contact with c, and makes contact with a. The current now flows from the battery, cell No. 1, through a to d, on to /, then along lever h to its centre, and down to post /, thence to line, and on to distant station, where it rings a bell; then to earth, and back again to central post p on switch, then to cell No. 3, and so completes the circuit. The "call" will be answered by the home bell ringing, produced by the same operation of pressing the button b at the distant station.

This sends a current into the line to the home station, which, entering the switch at /, passes upwards to the lever h, along this to /, then to d and c, and upwards to one of the top parts, then out to bell, which it rings in passing, and on to centre post p, and back through earth to distant station, and so again the round is completed. Now remove telephone t from the hook h; the spring . at once lifts up this lever, and through the insulated connection (a silk thread) n allows the spring g to make contact with j. Contact is also at the same time made by the lever with e, and broken at f. This operation brings the microphone or pantelephone into action.

Fig. 124.

The current now flows from No. 1 cell only, this being sufficient to work the local circuit through the microphone. Its course is through agj to pantelephone, then passing by k through the loose carbons m to o; up this it passes by a wire, then down on the other side to the induction - coil i, where it flows through the primary wire and back to the same cell No. 1. The secondary wire terminates in 2 binding posts on the top of the coil. This wire furnishes the current necessary to work the telephones. It starts, say, from the left - hand post and passes to e on switch, then along lever A, now in contact with e, and down to post f, thence through line to distant station, and through its telephone to earth, then back again it hastens to centre post p on home switch, and on through telephone to induction - coil, whence it started. The microphone bell and switch are best combined in one instrument. (Frank.)

(e) Switch for simplex Telephone (Fig. 125). - It is manipulated as follows: -