ON SOME MARVELS IN TELEGRAPHY. 249 



There is, however, another way in which an auxiliary wire 

 may be made to work. It may be so arranged that, when a 

 message is transmitted, the divided current flowing equally 

 in opposite directions, the instrument at the sending station 

 is not affected ; but that when the operator at the distant 

 station sends a current along the main wire, this neutralizes 

 the current coming towards him, which current had before 

 balanced the artificial current. The latter, being no longer 

 counterbalanced, deflects the needle; so that, in point of 

 fact, by this arrangement, the signal received at a station is 

 produced by the artificial current at that station, though of 

 course the real cause of the signal is the transmission of the 

 neutralizing current from the distant station. 



The great value of duplex telegraphy is manifest Not 

 only can messages be sent simultaneously in both directions 

 along the wire a circumstance which of itself would double 

 the work which the wire is capable of doing but all loss 

 of time in arranging about the order of outward and home- 

 ward messages is prevented. The saving of time is espe- 

 cially important on long lines, and in submarine telegraphy. 

 It is also here that the chief difficulties of duplex telegraphy 

 have been encountered. The chief current and the artificial 

 current must exactly balance each other. For this purpose 

 the flow along each must be equal. In passing through 

 the long wire, the current has to encounter a greater re- 

 sistance than in traversing the short wire ; to compensate 

 for this difference, the short wire must be much finer than 

 the long one. The longer the main wire, the more delicate 

 is the task of effecting an exact balance. But in the case 

 of submarine wires, another and a much more serious diffi- 

 culty has to be overcome. A land wire is well insulated. 

 A submarine wire is separated by but a relatively moderate 

 thickness of gutta-percha from water, an excellent con- 

 ductor, communicating directly with the earth, and is, more- 

 over, surrounded by a protecting sheathing of iron wires, 

 laid spirally round the core, within which lies the coppei 

 conductor. Such a cable, as Faraday long since showed, 



