Line Current Regulation in Bridge Polar Duplex 

 Telegraph Circuits 



By S. D. WILBURN 



Synopsis: A mathematical analysis of the bridge polar duplex telegraph 

 circuit, under the condition that the bridge arms are of equal resistance, 

 shows that there is a particular bridge arm resistance which results in maxi- 

 mum received current. As the bridge arm resistances are increased beyond 

 the value giving this maximum, the received current diminishes gradually. 

 On the other hand, as the bridge arm resistances are decreased below the 

 value giving the maximum, the recei\ed current drops off very rapidly. It 

 follows that when necessary to limit line current, the maximum received 

 current is obtained by placing the regulating resistance in the bridge arms. 

 Also when the line resistance is large enough to limit the line current to less 

 than the maximum allowable value, a gain may be obtained by increasing 

 the bridge arm resistance to the value which corresponds to maximum 

 received current. Experience has shown that in many situations where 

 difificulty is encountered in operating a duplex telegraph circuit with the 

 regulating resistances in the Hue, a very decided improvement is obtained 

 by transferring these resistances to the bridge arms. 



FOR the operation of polar duplex telegraph circuits, line batteries 

 of uniform voltage are generally used and it is usually desirable 

 to maintain the line current within fairly definite limits. The most 

 suitable line battery voltage and the desired limits for the line current 

 depend upon the type of line and apparatus used. In order to main- 

 tain the line current within the desired limits with uniform voltage 

 it is necessary to add resistance to the circuit in greater or less amounts 

 depending upon the length and gauge of the line circuit used. On 

 account of line trouble and the necessity for rerouting telegraph 

 circuits for other reasons, it is frequently desirable to switch a duplex 

 set from one line to another of difTerent resistance. To facilitate line 

 current regulation without delaying service when such changes in 

 line assignment are made, it is of considerable operating advantage to 

 include in the wiring of each duplex circuit an adjustable resistance 

 in the form of a rheostat mounted in an accessible location at the 

 duplex set so that the attendant can readily regulate the line current 

 at the time that necessary adjustments in the balancing artificial line 

 are made to suit the changed line condition. 



This paper outlines an investigation which was made with the object 

 of finding an arrangement of line current regulating resistance which 

 would result in the maximum steady-state received current with the 

 bridge duplex telegraph circuit shown by Fig. 1, where it is desired to 

 limit the line current to about .070 ampere. The condition for maxi- 

 mum steady-state received current was sought as the first step toward 

 determining the most suitable arrangement of the resistances with the 

 viewpoint that such an arrangement would probably be the most 



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