C. GENERAL PHYSICS. 169 



ductiou of more perfect appliances, since Gintel's and Sie- 

 mens and Halske's inventions. Transactions Hoy. Soc. of 

 jVeio South Wales, ISU, Vl. 



THE DOUBLE-BALANCE METHOD IX DUPLEX TELEGRAPHY. 



Louis Schwendler, electrician in charge of the telegraph 

 lines of India, has communicated to the Asiatic Society of 

 Bengal a memoir on the general theory of duplex teleg- 

 raphy, and has investigated with great detail the condi- 

 tions necessary to the successful use of the bridge method 

 under two suppositions : first, that the line is in perfect in- 

 sulation, and, second, that the line has appreciable leakage ; 

 the latter Jbeing of course the actual case in practice. He 

 first shows that, in order to obtain the best results in both 

 cases, the resistances and the currents must have certain re- 

 lations to each other, such that there should be two balances, 

 viz., a balance in one branch for the receiving current, and 

 a balance in another branch for the outgoing current. This 

 method elaborated by him, and which he calls the double- 

 balance method, fulfills the following conditions, which he 

 states are necessary, and are sufticient to place duplex teleg- 

 raphy on a par with single telegraphy. First, any varia- 

 tion in the resistance of the line has the least possible dis- 

 turbing effect on the receiving instrument. Secondly, any 

 disturbance can be eliminated by a single adjustment with- 

 out disturbing the balance in a distant station. Thirdly, the 

 maximum magnetic moment of the receiving instrument is 

 obtained and employed. Fourthly, a maximum working 

 current is available. This double-balance method was in- 

 troduced by him first in June, 1874, bcitween Bombay and 

 Calcutta, and has since then been working so satisfactorily 

 even during the worst time of the year, viz., the southwest 

 monsoon, that Colonel Robinson, director -general of the 

 telegraphs in India, has decided to introduce this duplex 

 method also on the other long main lines of India. e/bwr- 

 nal Asiatic Society of Bengal, 1874, 218. 



SCHWEXDLER's method IX DUPLEX TELEGRAPHY. 



In the annual address of the President of the Asiatic Soci- 

 ety of Bengal, it is said that the double-balance method in 

 duplex telegraph v, as invented and perfected by Mr. Schwend- 



H 



