There are two systems of duplex teleg raphy in use in this country and elsewhere, namely, the °Stearns) duplex and the °Polar° duplex, and these combined comprise the Edi son °Quadruple/co described later.
The Stearns Duplex.— The Stearns dn ple5c is operated by increment and decrement of current on the line, virtually as the single Morse system is operated, namely, by send ing a current over the line to actuate the distant relay, which attracts its armature; and by removing the current from the line, where upon the armature is withdrawn by its re tractile spring. The near or home battery is prevented from affecting the near relay by winding the relays differentially. The theory of the Stearns duplex is shown in Fig. 12, which represents the apparatus at two sta tions, A and B. The relays M, M', are wound with two coils in opposite directions around the cores; one of the coils of each relay being connected to the main line and the other to a rheostat, or resistance coils, R, R'. K and K' are transmitting keys. When key K is closed it puts a positive electromotive force to the line; when key K is closed it puts a negative electromotive force to the line. When the keys are •open) the line wire is placed to ground. Since the battery current will di vide between the two coils of the relay in proportion to the resistance of the circuits attached to each coil, a means must be pro vided to make those circuits of equal resist ance, otherwise unequal currents will flow through the coils and the relays will be oper ated by the home battery. It is the function of the rheostats R to provide a resistance equal to that of the main line and the main line coil 1, of the distant relay, so that there wiH be a °balance." In the figure, key K being closed and key K' open, a current flows as indicated by the arrows from A to B. As equal currents are flowing in opposite direc tions around the core of relay M its armature is not attracted. On the other hand, as there is a direct path for the current at B to earth, via the back contact of key K', no current from A will flow through coil 2 of relay M'. Consequently, the core of M' is magnetized and attracts its armature a' as shown. If now key K' should be closed this would add 100 volts negative polarity to the main line circuit, making 200 volts on the main line, while it would place only 100 volts to the arti ficial line coil at B. Hence double the amount of current will flow in the line coils 1, of each relay, that flows through the artificial line coils 1 and 2 Thus the armature a' of M' is now attractea and armature a of M continues to be attracted.
The Polar Duplex.—This system also em ploys differential wound relays, transmitting keys and main and artificial lines similar to those of the Stearns duplex and for the same purposes, but the type of transmitting keys and relays is different in the two systems.
The relay employed in this system is termed a polarized relay (E, E', Fig. 13) and is based on the fact that unlike magnetic poles attract one another, and that like poles repel one an other. Then if one pole of a bar permanent magnet a, say its north pole N, be so pivoted that it may move freely toward the south pole ; of an electromagnet E, it will follow that by changing the direction of the current flow ing in the coils of the electromagnet, the per manent magnet a will oscillate between the poles in response to the changes of polarity in the electromagnet. If the permanent magnet a, which is virtually the armature of a polar ized relay, be given control of a 'local circuit relay records them. It is only necessary to pro.. vide a pole-changing key F F' to reverse the direction of the current flowing in the line, to bring about this result. Fig. 13 represents in theory the circuits and apparatus of a differ ential polar duplex; the apparatus and arrange ment of circuits at the terminal stations A and B being indicated. When the °pole-changer" F is as at A, a positive pole of a battery is placed to the line and the direction of the current is as indicated by the arrows. When the key F' at B is closed, a negative pole of a battery is placed to the line. In these positions of the keys the direction of the cur rents in the main and artificial lines is as shown by the arrows, and the relays at both ends of the line are recording a signal, the sounders being attracted. This is due to the fact that an excess of current is flowing in the line coils of each relay in a direction to produce magnetic poles in the cores of the relays as marked. If nciw, for instance, the key F' at B should be opened the effect would be to place a positive pole of the battery to the line. The effect of this is that each end of the main line is placed at equal and like potentials, and herice-no cur rent flows over the main line or in the 'main line coils of the relays. A positive current from the battery at A with a strength' of say 1 will still flow through artificial, line coil at A, but in a direction which 'Will teverbe the previous polarity of the core; consequently the armature a of the relay is attracted to the left side and the local circuit of sounder D is opened. An examination of the conditions Will show in every case that when the pole-changer at either end is opened or closed, the local circuits controlled by the armature of the dis tant relays will also be °opened" or "closed." From which it follows that dots and dashes may be transmitted from both ends of the line simultaneously by a proper manipulation of the pole changers.