Mr. 0. Heaviside on Duplex Telegraphy. 431 



terval of disconnexion, but did not find it necessary to introduce 

 equivalent resistances when they were out of circuit, though 

 theoretically this should be done, and the same key could be 

 made to do it. 



The last plan will be easily recognized to be based on the 

 method of comparing electromotive forces known as Poggen- 

 dorff's compensation, in which the battery having the lesser 

 electromotive force is not allowed to act. The other plan (fig. 3) 

 is also an adaptation of a method of comparing the working 

 electromotive forces of batteries, which I devised three years ago, 

 and subsequently published in the ' English Mechanic ' for July 

 5, 1872, No. 380, p. 411. My only reason for mentioning this 

 is, that it is claimed by Emile Lacoine as a " new method of 

 determining voltaic constants," in the Journal Telegraphique, 

 vol. ii. No. 13. See also the { Telegraphic Journal 3 for April 

 this year. 



The greatest drawback to duplex working (and this is common 

 to all known systems) is the changeability in resistance of the 

 line- wire itself, caused by defective insulation, variations of tem- 

 perature, &c. ; and in such a wet and changeable climate as ours, 

 this fixes a limit to the length of line on which a duplex system 

 can be worked with advantage, making it less than can be worked 

 through in the ordinary manner. On shortlines the resistance never 

 varies much in any weather (unless actualfaultsoccur),and it is not 

 necessary to vary the balancing resistances. But on long lines this 

 variation is sometimes very considerable ; and it is questionable 

 whether, in the present state of telegraphy, a long circuit in this 

 country, as from Glasgow to London, could be profitably worked 

 in wet weather. But the variations in the resistance of submarine 

 cables (having no land-lines attached) are so very much less, 

 that it seems probable, a priori, duplex telegraphy would be suc- 

 cessful with them. Of course their electrostatic capacity must 

 be balanced by condensers. It could also be applied to the 

 system by which some long cables are worked, where there is 

 no metallic circuit through the receiving instrument, which is 

 placed between a condenser and the cable. 



Those systems where the out-going currents do not pass 

 through the receiving instruments have a peculiar and perhaps 

 what will some day (when telegraphy, now in its infancy, has 

 arrived at years of discretion) be considered an important ad- 

 vantage over the differential system. It is theoretically possible 

 to send any number of messages whatever simultaneously in one 

 and the same direction on a single wire. Now by combination 

 with a " null " duplex system it becomes obviously possible to 

 send any number of messages in the other direction while the 

 opposite correspondences are going on, and without interference. 



