426 Mr. 0. Heaviside on Duplex Telegraphy. 



pressure. With constant temperature, however, according to 

 rny earlier experiments, <j> augments as the pressure diminishes. 

 Consequently for the same limits of temperature 



( d l\ > ( d ^\ 

 \dtJ p \dt/ v ' 



Hence the value of a is universally greater for constant pres- 

 sure than for constant volume. 



Physical Laboratory of the Polytechnic, 

 Aachen, June 5, 18/2. 



LIII. On Duplex Telegraphy. By Oliver Heaviside, Great 

 Northern Telegraph Company, Newcastle- on-Tyne*. 

 [With a Plate.] 

 T^UPLEX TELEGRAPHY, the art of telegraphing simulta- 

 J-/ neously in opposite directions on the same wire, which was 

 first performed by Dr. Gintl in 1853, and subsequently engaged 

 the attention of so many inventors, until lately seemed never 

 likely to be carried out in practice to any extent. According to the 

 very practical author of ' Practical Telegraphy/ " this system has 

 not been found of practical advantage;" and if we may believe 

 another writer, the systems he describes " must be looked upon as 

 little more than feats of intellectual gymnastics — very beautiful 

 in their way, but quite useless in a practical point of view." 

 However, notwithstanding these unfavourable reports as to the 

 practicability of duplex telegraphy, the experience of the last 

 y ear has negatived them in a striking manner, and made the so- 

 called "feats" very common-place affairs. Circuits worked on 

 a duplex system are now established in various parts of the 

 United Kingdom — not to mention the United States, where the 

 resurrection of these defunct schemes took place — and continue 

 to give every satisfaction. There seems little reason to doubt 

 that this system will eventually be extended to all circuits of not 

 too great a length, between the terminal points of which there 

 is more than sufficient traffic for a single wire worked in the 

 ordinary manner — that is to say, only one station working at a 

 time. 



I propose in this paper to give a short account of the theory 

 of duplex telegraphy by the principal methods, and to describe 

 two other methods, which are, I believe, entirely original. 



To begin at the beginning. Prior to 1853, it is said to have 

 been the current belief of those best qualified to judge, that to 

 send two messages in opposite directions at the same time on a 



* Communicated by the Author. 



