70 



NATURE 



[May 27, 1875 



struction. It is not, however, only by the automatic 

 process that the full transmitting capacity of a con- 

 ducting wire can be attained. Metallic conductors under 

 certain conditions are capable of transmitting more than 

 one current at the same instant of time, both in the same 

 and in opposite directions ; and by a very ingenious 

 system of adjustment of electric resistances and balance 

 of currents, perfected by Messrs. Stearns, Edison, and 

 Prescott, the American electricians, intelligence can be 

 transmitted and recorded over a single wire in opposite 

 directions at the same moment. This system of trans- 

 mission is known as " Duplex" and " Quadruplex " Tele- 

 graphy, and is already extensively employed by the West- 

 ern Union Telegraph Company in the United States, and 

 over several of the more important circuits in Great Britain. 

 The " Duplex " system is working in America between 

 nearly all the principal cities, and has recently been in- 

 troduced between Port Hastings, on the island of Cape 

 Breton, where the land circuits are in connection with 

 the submarine cables, and San-Francisco, a distance 

 little short of 5,000 miles. The " Quadruplex " system 

 has been successfully introduced between New York and 

 Boston, with a transmitting capacity upon a single wire 

 equivalent to the transmitting power of four wires worked 

 upon the ordinary Morse system. Thus, by employing 

 arrangements such as the " Duplex " and " Quadruplex," 

 a circuit may be worked either as one wire, or two, three, 

 or four wires, according as the transmitting capacity of 

 the circuit may require to be increased. 



As is well known, several sounds may be conveyed at 

 one and the same time by vibrations through a rod with- 

 out interference, and it is difficult to realise the accuracy 

 with which every vibration is reproduced by anyone who 

 has not witnessed an illustration of the "transmission of 

 sound " by solid conductors. So it is with " Duplex " and 

 " Quadruplex " transmissions through the same wire in 

 opposite directions at the same moment of time ; it is 

 equally difficult to reahse how distinct signals can be 

 received at either end without interferingwith or destroying 

 each other ; and yet the principles involved are very 

 simple and easy of explanation. 



By the Duplex system, one of the most difficult problems 

 incident to the successful development of telegraphic lines 

 has been solved, namely, how to provide for the annual 

 increase (averaging 20 per cent.) in the amount of busi- 

 ness without the annual expenditure on capital account 

 for the erection of additional wires. In the United States, 

 over 150,000 miles of wire are in operation, the rate of 

 increase being something like 20,000 miles per annum, 

 and the Duplex system is capable of doubhng the carrying 

 capacity of these wires. The great value of the Duplex 

 system consists in its capability to double the capacity 

 of a wire at any moment, should injury by storm or 

 conflagration interrupt the circuits. By its means, the 

 moment one wire is restored to continuity it becomes 

 equivalent to two, and a second wire raises the carrying 

 capacity of the circuits to four wires, and by skilful 

 manipulation the system may be introduced and adjusted 

 on a circuit in about a minute. From the earliest 

 days of telegraphy it has been well known that two 

 currents, either in the same or in opposite directions, 

 could be passed simultaneously through a conducting 

 wire ; indeed, by this means, often has the frame of 

 mind and temper of the operator at the distant station 

 been clearly read at the receiving station, even though 

 situated some hundred miles distant. When the direc- 

 tion of the currents from the two stations are passed 

 into the wire in the same direction, the directive force 

 of the needle becomes more decided, and when the 

 direction is contrary the motion of the needle will be 

 comparatively neutralised and scarcely perceptible. The 

 effect of a current transmitted along a wire from one 

 station upon a galvanometer needle while currents are 



being transmitted from another station has therefore been 

 long known. How this circumstance has been applied to 

 the indication of distinct signals will now be explained. 

 Let us suppose two stations, A and B, are to be connected 

 for signalling each other upon the Duplex system : the 

 action of the coils in the instruments at the respective 

 stations is so arranged that neither station's local or out- 

 going current shall affect its needle when passed into the 

 line, its dial being left free to indicate the effects pro- 

 duced by the incoming current from the distant station. 

 For this purpose it is necessary to wind the coils of the 

 instruments with two parallel wires after the manner of a 

 differential galvanometer. Now, as is well understood 

 in testing a line wire for resistance between two stations 

 with a differential galvanometer, until the artificial resist- 

 ance interposed has been made equal to that of the line 

 to be tested, the battery current passed by the key into 

 the galvanometer will move the needle in the one direction 

 if the artificial resistance is too small, and in the other 

 direction if the resistance is too great. It is only when an 

 accurate balance is obtained — that is, when the two 

 resistances have been made equal — that a current will not 

 move the needle, because then the current is equally 

 divided between the coil connected with the artificial 

 resistance and that connected to the line, which two 

 coils being wound in opposite directions counteract one 

 another. Thus, so long as the artificial resistances (rheo- 

 stats) at each end of the line are equal to that of the cir- 

 cuit, each station will see the current sent by the other, 

 while neither station will see upon his own instrument the 

 current he is passing into the line ; and for this reason, that 

 the currents sent by each station divide equally between 

 the line and the rheostat, passing through the coils in 

 opposite directions, and have therefore no effect upon the 

 needle of the sending instrument. When the distant 

 station sends a current, it either increases or diminishes 

 the effect of the home current ; in the first case, it aug- 

 ments that portion which passes through the coil con- 

 nected to the line, so that more flows into the line than 

 into the rheostat, and the needle moves. In the second 

 case, it reduces the current flowing to the line, and more 

 will flow through the rheostat, moving the needle in an 

 opposite direction. Thus it is seen that the two currents 

 do not pass one another, but that when both stations 

 signal at the same time, the current sent by either of them 

 acts upon the distant instrument by determining whether 

 the currents sent by that station shall pass through the 

 line or the rheostat. Thus we see that when station A 

 signals separately, the current is equally divided in pass- 

 ing through its instrument coil, and its effect is neutralised 

 upon the needle, but it passes through both coils of the 

 distant instrument in the sajne direction, and therefore 

 produces a signal. If both A and B depress their contact 

 keys at the same moment, the currents from the two bat- 

 teries are united so far as the line wire is concerned, and 

 this produces an effect upon the differential arrangements 

 at each equivalent to a lessening of the resistance of the 

 line, and therefore more current flows to the line than 

 through the rheostat. It is thus seen that the Duplex 

 system affords a means of increasing the transmitting 

 capacity of a wire ; and an invention which practically 

 converts one wire into two, three, or four, as the necessi- 

 ties of business may require, is of great value. 



A short historical summary of the introduction and 

 progress of the electric telegraph, from its earliest 

 appUcation in a practical form to the estabhshment 

 of its present world-wide reputation and utility, will 

 be naturally of interest to the general reader ; and the 

 following short sketch may convey in a succinct manner 

 the step by step progress that year by year has registered 

 the index of improvement. It is not intended in any way to 

 make the present sketch personal: some well-known names 

 must of necessity be referred to, and the reader should also 



