118 Mr. L. Schwendler on the General Theory 



early as 1849 Messrs. Siemens and Halske, of Berlin, took out a 

 patent in England* for the simultaneous transmission of a plu- 

 rality of messages by a suitable combination of wires; and 

 although this patent does not refer directly to duplex telegraphy 

 as it was subsequently understood, it must, notwithstanding, 

 be regarded as a forerunner of it. In point of fact, Dr. W. 

 Siemens's idea represents the general problem of which duplex 

 telegraphy is only a particular case. 



In 1854 Dr. Gintl, of Vienna, tried his "compensation" 

 method of " duplex" working between that capital and Prague f, 

 and on the 30th November of the same year read a paper before 

 the Kaiserlich-konigliche Academie of Sciences of Vienna J on 

 the practical solution of the same problem by employing a Bain's 

 electrochemical telegraph-apparatus instead of a Morse's receiv- 

 ing instrument. 



In the. summer of 1854, after Dr. GintFs experiments be- 

 tween Vienna and Prague had brought the subject prominently 

 to notice, Messrs. Siemens and Halske, of Berlin, and Herr 

 Frischen independently, invented the " differential " method. 



In January 1855 Ecllund§ made experiments on the line be- 

 tween Stockholm and Gothenburg. He employed a " differen- 

 tial " method, which he had invented in 1848, for the purpose 

 of measuring accurately Faraday's "extra currents." 



In papers read at Paris on the 16th July and 6th August, 

 1855 || , before the Academy of Sciences by M. Zantedeschi, he 

 claims the honour of having first suggested the idea of duplex 

 telegraphy ; for as early as 1829 he had proved the possibility 

 of the simultaneous transmission of currents in opposite direc- 

 tions through a single conductor. Having never seen his ori- 

 ginal communication of 1829, it is impossible for me to say how 

 far these early ideas of Zantedeschi bear on the problem ; but it 

 is certain that both he and Dr. Gintl took a great deal of trouble 

 to prove an erroneous theory, viz. that two distinct electrical 

 currents can pass simultaneously in opposite directions through 

 the same conductor without in any way interfering with each 

 other. Such a supposition is in direct opposition to the elec- 

 trical laws which were already known in 1829 If, and besides 

 is in no way required in order to explain the simple pheno- 

 menon of duplex telegraphy**. 



* 23rd October, 1849. The actual wording of the English patent is 

 unknown to me. 



t Polyt. Centralbl. 1853, p. 1475. 



1 F/ien. Akad. Sitzungsber. xiv. 



§ Pogg. Ann. 1856, vol. xcviii. p. 634. || Ibid. p. 123. 



% Ohm published his classical work Die galvanische Kette matliemutisch 

 bearbeitet in the year 1828. 



** Dr. W. Siemens, Pogg. Ann. vol. xcviii. p. 123. 



