2 DUPLEX TELEGRAPHY. 



proper adjustment. To overcome this difficulty, tlie resistance 

 must be gradually increased until the two currents are equalized 

 so that the closing of the key has no effect on the armature of 

 the relay, and no signals are made except at the distant receiving 

 station. It is therefore evident that when station A is sending 

 to station B, A's relay is unaffected, while the relay at B will be 

 made to work and the instrument record A's signals, and vice 

 versa. Stern's duplex system, lately perfected in America, and 

 now daily attracting the attention of telegraph engineers in most 

 parts of the world, is on a principle which is dependent on 

 producing an equality of tensions which are sometimes called 

 potentials ; but to be more explicit to those who have not studied 

 the science of electricity, it is, according to Jenkin, the difference 

 of electrical condition in virtue of which work is done, by moving 

 from a point at higher tension to that of a lower ; or perhaps Mr. 

 Preece has made it clearer by calling it an analogous term to 

 pressure, as applied to fluids and gases. With this system the 

 relay coils are also wound with double wires ; it has proved a 

 great success, partly by improvements on old principles, but 

 more particularly in consequence of the introduction of more 

 perfect appliances siace Gintl's, and Siemen's, and Halske's 

 inventions. The arrangement before you this evening is perhaps 

 the most simple duplex working apparatus yet devised ; the two 

 instruments are the ordinary Morse recorders with relays in 

 every-day use on the lines in this Colony. The relays are wound 

 with one continuous wire, and the only additions are the two 

 vertical water columns for producing the necessary artificial resis- 

 tance for dividing the currents ; the actual line resistance being 

 equal to 150 miles of wire. 



With this plan only one battery is employed for the line circuit 

 which is continuous, that is, in one direction, and the adjustment 

 requires very little attention. It must not be understood that 

 two distinct currents of electricity pass in opposite directions 

 through one line of wire at the same time ; but the problem has 

 been solved by increasing the amount of current on the main 

 circuit when signals are sent simultaneously. ' Although duplex 

 telegraphy may now be considered beyond a doubt as to its 

 practicability, we are not likely to remain satisfied with its 

 success for any lengthened period. Mr. Meyar's ingenious 

 invention of the multiplex telegraph, exhibited in the late Vienna 

 Exhibition, bids fair to eclipse all the telegraph instruments now 

 'in use for rapidity of signalling through one wire. This instru- 

 ment has already been tried between Paris and Lyons, with 

 four transmitters on one line, and 100 to 120 messages of average 

 length are sent through per hour. 



