The telegraph line extends also from earth. E' at X, to at 1", one set of instruments being shown in detail at each end. The circuit may be traced as follows, the oper ator at X being supposed to be send ing, and the operator at }'receiving : From earth, E', through the line battery positive to line, transmitter contact, 1', switch, d', segment No, 1 of the distributor, and through the trailing brush to the large segment of the distributor, to which the line is connected at X; thence over the telegraph line, T 1,, to the distributor parts at E out through the switch, transmitter back contact, receiving relay IV, and completing the circuit at I.
The speed is so regulated in practice as to give each instrument, when its circuit is closed, 30 contacts with the line per second, which, admitting four contacts per revolution, would mean an average speed of 71- revolutions per second, or 450 per minute. But as the dis tributor motors (Fig. 7) move at half the speed of the controlling motor in the middle of the line, this one, which carries the pole changer, is driven at about 900 revolutions per minute, producing therefore 1,8G0 reversals of current per minute on the synchronizing line. and a corresponding number of vibrations of the polarized relay armatures. But as four of these are required to produce a single revolution in the armatures in the distributor motors, their speed is brought to about 450, as stated.
The Field Sextuplex Telegraph.—An ingenious improvement in multiplex telegraphs is that of Mr. S. D. Field, operating as a sextuples. Three different qualities of cur rent are employed, viz.: a direct current of increasing and decreasing strength, operating a neutral relay ; a reverse current, operating a polarized relay ; and a rapid vibratory cur rent, which sets a telephonic diaphragm in rapid vibration. These three currents acting upon corresponding receiving instruments, do not interfere with each other, as will be shown below ; and as each one type of working is duplexed by the well-known compensating method, the line is evidently capable of transmitting three messages in either direction, or six simultaneously.
The arrangement of circuits and apparatus by which these results are effected is shown in the accompanying diagram, Fig. 8. Both the main line and locals derive current from a dynamo. The latter is shown at F, and the armature, as will be seen, is provided with two independent sets of windings, which deliver current respectively to the commutators, E and D. The local currents are taken off the commutator, F, the circuit connecting with the three local transmitters, 7..?, and 3, which are mauipulated in the ordinary way by the keys, K', .1173. The main current is taken from the armature from the commutator, D, this cur rent serving to actuate the neutral and polarized relays, which are shown diagrammat ically at Nand respectively. It will he noticed that the dynamo, F, is shunt-wound.
Its armature is of 110 ohms resistance, and it has an E. M. F. of 390 volts at 500 revolutions. The shunt coil is divided so as to give a long and a short shuntat the points, G, IT, depending upon whether the transmitter 2 be closed or open. The resistance of the short shunt is 540 ohms, and that of the long shunt is 6,000 ohms. Hence it follows that by pressing the armature of transmitter f is attracted to the front stop, and short-circuits the long shunt of the dynamo. This, of course, causes an increase of current in the short shunt, the strength of the field magnets remaining constant ; and hence there ensues a de creased effect in the line current, and it is upon this increase and decrease of the direct cur rent that the neutral relay;: operates.
Transmitter 1 operates a pole changer, by which reverse or alternate currents are sent over the line, which actuate the polarized relay shown diagrammatically at T. The pole changer is so adjusted as to be continuity-preserving as regards the line, but with a very slight break toward the dynamo.
It is evident that the continuous current designed to operate the neutral relay has no effect upon the polarized relay ; but the reverse currents designed for the latter would affect the neutral relay if some provision were not made to prevent this disturbance. This has been recognized by Mr. Field, and he overcomes the difficulty in a very simple manner.
The neutral relay 2 is shown in part perspective in Fig. 9. To understand its operation, we will premise that when ordinary reverse currents are sent through a neutral relay the armature is kept in a state of vibration, breaking contact momentarily at each reversal, but being immediately re-attracted. With the arrangement of the neutral relay shown in Fig. 9, the reverse current has no effect on the armature. This result is obtained by taking advantage of the induced currents generated by the re versals. As will be seen, the etre of the relay is lengthened, and has a bobbin, B. surrounding it. The latter is connected to another small bobbin, C, surrounding a core, H, which is placed opposite a small cylinder of iron, Ti, acting as an armature and attached to the lever of the relay. The reversal of current in the relay bobbin causes a change of polarity in the core, and the tendency is to momentarily throw off the armature ; but at the same instant of the reversal of polarity an induced current is set up in the bobbin, 13, which is in opposite direction to the primary, and which, in circulating through C, tends always to magnetize the core, II, oppositely to that of the main core, and hence, with a corresponding influence upon the small armature, /I. The result of this is, evidently, that with two opposite influences act ing upon the lever, it will remain stationary and insensible to the effects of the reverse currents.