Flow of Electricity in a uniform plane conducting Surface. 475 



By February next duplex working will therefore have been sub- 

 mitted to a most severe test, applied as it will have been for a whole 

 year to a long line the electrical condition of which is highly vari- 

 able with respect to season and locality ; and its practicability will 

 doubtless again be clearly proved, as has already been the case on 

 the Calcutta-Bombay line (1600 miles), where, under no more 

 favourable climatic conditions, duplex has for the past twelve 

 months not only fulfilled but surpassed the expectations formed of 

 t. No difficulties have been experienced, and, it is believed, never 

 will be. 



Strange as it may appear from a theoretical point of view, it will 

 nevertheless be found in practice that a line worked duplex carries 

 more than double the traffic of the same line worked singly ; for it 

 represents two lines carried on different posts far distant from one 

 another, instead of two parallel lines on the same posts ; and con- 

 sequently the highly injurious effects of voltaic induction are eli- 

 minated. 



Further, the receiving signallers, not being provided with keys, 

 are unable to interfere with messages during their transmission. 



Corrections and repetitions do not necessitate a stoppage of work ; 

 for they are obtained in the following manner : — the receiving sig- 

 naller marks with a cross or underlines the words to be repeated, 

 and places the message by the side of the sending signaller, who 

 calls for the repetitions directly he has finished the message he is 

 transmitting ; and during this call the distant station may either 

 send fresh messages or may also call for repetitions. Consequently 

 single working need never be resorted to, and the simultaneous ex- 

 change of messages and corrections becomes continuous. 



The Indian system of receiving (the sounder system which has 

 now been universally recognized as the only right one for hand- 

 signalling) thus necessitates constant attention on the part of the 

 receiving signallers ; for any inattention on their part at once be- 

 comes known to the controlling officer. 



LVI. On the Flow of Electricity in a uniform plane conducting 

 Surface. — Part II * By Professor G. Carey Foster, F.R.S., 

 and Oliver J. Lodge, B.Sc.f 

 43. "FN an unlimited uniform conducting surface, at given 

 -1- points of which there are sources and sinks of electri- 

 city, the electrical condition of each point within a limited area 

 must be determined by the electrical condition of its boundaries. 



* For Part I. see Phil. Mag. S. 4. vol. xlix. pp. 385-400 and 453-471 

 (May and June 1875). 



t Communicated by the Physical Society- The substance of this paper 

 was read to the Physical Society on the 27th of February, 1875; but it 

 contains also experimental results obtained since that date. 



