532 Mr. Sidney George Brown [March 12, 



is, I think, the best to employ, ])ecause it gives, in practice, higher 

 speeds than any other form of bridge. 



The receiving instrument is joined to the commencement of the 

 cables, and is thns not interfered with by the sending currents, 

 because there is no tendency for the current to flow, one way or the 

 other, the real and artificial cables having exactly the same electrical 

 properties and acting on the sending current in the same way. But 

 the current that is received floAVS only from the real cable, and is not 

 balanced by any from the artificial, so that the receiving instrument 

 is w^orked by it. 



When duplex is properly adjusted it is said to be in balance, 

 from its similarity to the adjustment of an ordinary balance used 

 for weighing goods. 



Take the ordinary balance as an illustration of the electrical one : 

 Let one scale-pan represent the cable, the other the artificial ; if 

 equal weights are placed in each pan the beam will not turn, but the 

 beam will turn if, while equal weights are or are not in the pan, a 

 small weight is added or placed on one pan. 



In the cable " duplex," the receiving instrument will not be 

 affected by the sending current because the voltage is always the 

 same on each side of the instrument, but will turn to indicate a 

 signal when a voltage is received or is added to or subtracted from 

 the voltage already on the cable side, due to a voltage being applied 

 to the cable at the far end. 



In Fig. 6 is shown the simplest diagram of a cable "duplex," 

 and Fig. 7 illustrates its mechanical equivalent ; the lettering is 

 similarly related. 



If the battery B sends equal currents into cable and artificial 

 line, as it should do if there is a perfect balance, no current will 

 flow through S, and thus the receiver S is unaffected by the sending 

 voltage. Or if the pans of the balance have equal weights B placed 

 on them, the indicator S will not move. On the contrary, if a 

 voltage is received from the cable C, this voltage is added to or 

 subtracted from whatever voltage may be in C at the time, due to 

 the sending battery, and thus there will be a difference of potential 

 across S, and the receiving instrument will be worked from cuiTents 

 sent from the far end of the cable and from these currents only. 



In the mechanical analogy a small weight W is added to or taken 

 from one of two equal weights in the pans C and A L, and the beam 

 will be tilted and will be moved by this weight only, however the 

 weights B B are varied. 



The voltage of the battery as applied to the sending end of a 

 cable is very much greater than that received from the cable to work 

 the instrument, say in the relation of 40 volts to ^^ volts in the case 

 of a moderately long cable, or as 800 is to 1, and the sending and 

 received currents resulting from the same follow a similar pro- 

 portion. 



