398 Trmisactions . — Chemistry/. 



I have endeavoured, in the foregoing, to render intelligible how two 

 currents circulating in two conductors parallel to each other can continue to 

 do so without interfering with each other when connected, provided certain 

 conditions are observed. The attached diagram (PI. XXVI., fig. 4) shows the 

 exact proportions in a duplex circuit on the principle of the Wheatstone 

 Bridge, and is also a fac-simile of the plan, both as regards the relative values 

 of the different resistances and the position of the apparatus, upon which 

 duplex telegraphy is conducted in one of the wires in the Cook Strait cable, 

 now in successful operation since the 18th June last. 



It will be seen on reference to the diagram that the same proportions, as 

 regards units of resistance, are employed at either end of the duplex circuit, 

 the letters A and B representing the two ends of the cable and the apparatus 

 employed at each end. If the an*angement at one station (A, for instance) is 

 explained, the same may be considered as representing the conditions that 

 exist at B. 



The battery at A consists of ten cells (modified Daniell's Gravity Battery) 

 of an internal resistance equal to 100 units or thereabouts. The copper pole 

 is connected to the key, and the zinc pole is placed direct to earth. When the 

 key is depressed the copper current flows, one portion passing to line and cable 

 through the resistance of 800 units, and the other to the artificial resistance 

 (520) through the resistance (400). The proportions in the bridge are 800 on 

 the one side and 400 on the other, so that when the current arrives at H it 

 divides in proportional pai*ts, two-thirds of the current passing by way of H D 

 through the artificial resistance to earth, and the remaining one-third by way 

 of H C to the line, and after traversing the line and cable arrives at E, and 

 then splits again, two-thirds passing by way of E F to earth, and the remaining 

 one-third by way of E B to earth. The two-thirds that passes through E E 

 works the relay, and thus records the signal that is sent from station A. 

 Turning again to station A, it will be seen that the out-going currents, as 

 I said before, divide proportionally at H, two-thirds passing by way of H D, and 

 one-third by way of H C ; and as these proportions are still maintained beyond 

 the points C and D on either side of the relay, there is no temptation for the 

 current which traverses H C to pass through C D, nor is there any temptation for 

 the current which traverses H D to pass through DC; for as the currents in 

 each of the above circuits are proportional throughout, and the i^otcntial of each 

 current the same at C and D, consequently they exercise equal and opposite 

 efiects on the relay in C and D, and therefore when the currents are circulating 

 in H C and H D, their potentials being the same, there is no current in C D ; 

 thus the first step towards cficcting duplex has been accomplished — viz., we are 

 enabled to send the home current to lino without aflfecting the home-receiving 

 apparatus. But supposing, instead of the artificial resistance (520 units), we 



