File:EB1911 Telegraph - Duplex Working - bridge method.jpg

English: The “bridge” method for duplex working of a telegraph line: instead of sending the currents through the two coils of a differentially wound relay or receiving instrument as in Frischen's method, two resistances a and b are inserted, and the receiving instrument is joined between P and Q. The currents thus divide at the point D, and it is clear that if the difference of potential between P and Q is unaffected by closing the sending key, then no change of current will take place in the instrument circuit. The relative potential of P and Q is not affected by the manipulation of the sending key if the resistance of a bears the same proportion to that of b as the resistance of the line does to that of the resistance R; hence that is the arrangement used. One very great advantage in this method is that the instrument used between P and Q may be of any ordinary form, i.e. relay, Hughes, siphon recorder, etc.