106 PROCEKDINGS OF THK CANADIAN INSTITUTE. 



In this paper I de.sire to call the attention of the members of the 

 Institute to some recent advances that have been made in the direc- 

 tion of duplexing telephone lines. It is now ten years since the 

 telephone was introduced but it is only quite recently that attention 

 has been directed to duplex telephony. Duplex telephony seems to 

 be adapted to long distance telephony only. In long distance tele- 

 phony the best results ai'e obtained when two wires are used, one 

 wire being used as a return wii'e ; — the two wii'es forming what 

 is called a metallic circuit. The wires used for this purpose require 

 to be first-class conductors. They are now made of copper. This 

 necessarily very miich increases the cost of long distance telephone 

 lines, firstly because copper wire is expensive and secondly because 

 two wires instead of one ai'e used. 



Hence any means oi" device that will increase the working capacity 

 of telephone-trunk-lines will proportionately decrease the cost of 

 long distance communication by telephone. 



When I took up this question a little over two years ago my 

 object was to use the metalKc circuit or double wire trunk line sys- 

 tem connecting two telephone exchanges so that either two telephone 

 subscribers or a telephone subscriber and a central olfice operator at 

 each end of the metallic circuit could communicate simultaneously 

 without interference. In order to accomplish this I found that there 

 were two problems to solve, firstly, how to render the centi-al ofiice 

 receiving telephones neutral to the subscribers' transmitters, and 

 secondly, how to render the subscribers' receivers neutral or irrespon- 

 sive to the central ofiice transmitters. 



The solution of the first problem was not attended with any great 

 difficulty. The plan adopted was as follows, namely : — At each central 

 ofiice and at each end of the metallic circuit a subscriber's line was 

 attached. Instead of using the ordinary telephone receiver with a 

 single coil, a telephone with a double coil was used and instead of 

 inserting said coil in a single branch of the metallic circuit, one coil 

 was inserted in one branch and the other coil in the other branch of 

 the metallic circuit and the connections wei'e made so that electrical 

 currents traversing one coil, would be neutralised by the electrical 

 currents traversing the other coil. Speaking technically, the coils 

 were connected difierentially. The electrical currents generated by 

 the subscribei's' ti'ansmitters passed through the two coils in opposite 



