of Duplex Telegraphy, 125 



a=d=f=g=w + ft 

 L 



"* | 



It will be clear that the given solution fulfils the following 

 conditions, which are necessary and sufficient to place duplex 

 telegraphy on a par with single telegraphy. 



i. Any variation in the resistance of the line has the least pos- 

 sible disturbing effect on the receiving instrument. 



ii. Any disturbance can be eliminated by a single adjustment in 

 the b branch without disturbing balance in the distant station. 



iii. Maximum magnetic moment of the receiving instrument. 



iv. Maximum current. 



There seems to me to be no other method that can fulfil all 

 these conditions simultaneously; and the "double-balance"* 

 method must therefore be pronounced perfect in every conceiv- 

 able respect. I am convinced that if the general problem of 

 duplex working were investigated by means of the variation 

 calculus, the double-balance method would come out as the final 

 and only solution f. 



Addendum — Historical. 

 When reading this paper before the Asiatic Society (on the 4th 



* I have called this method the " double-balance " method, since there 

 are two balances to be fulfilled in each station — namely, balance in the b 

 branch for the arriving current, and balance in the g branch for the outgoing 

 current. 



t The double-balance method was introduced on one of the important 

 Bombay-Calcutta main lines in June last. Since then this duplex method 

 has been working so satisfactorily and with such regularity and speed, even 

 during the worst time of the year (south-west monsoon), when necessarily 

 the insulation as well as the inductive capacity of lines are so enormously 

 variable, that about its thorough practicability no doubt can be entertained ; 

 and Colonel Robinson, Director-General of Telegraphs in India, has conse- 

 quently decided to introduce this duplex method also on the other long 

 main lines of India. 



At present the apparatus for the Bombay-Madras line (worked direct 

 800 miles) is almost finished, and the apparatus for Calcutta-Rangoon is 

 under manufacture. 



The Calcutta-Bombay main line is worked duplice with Jabalpur only 

 in translation ; distance between Calcutta and Jabalpur 850 miles ; distance 

 between Jabalpur and Bombay 640 miles. The wire is almost throughout 

 No. 5| B. W. G. (diam. =5| millims.). 



This experiment, on such a large scale and made under the most unfa- 

 vourable meteorological conditions, has proved to evidence the practicabi- 

 lity of the double-balance method, which certainly will invariably succeed 

 on any line where single telegraphy is possible. 



