632 BELL SYSTEM TECIIXICAL JOURNAL 



From this equation, it is found that the bridge arms, each consisting 

 of a 500 ohm bridge coil only, as shown by Fig. 1, are too small for 

 maximum received current if the line resistance is greater than ap- 

 proximately 555 ohms. With line circuits ranging in resistance from 

 1000 to 2500 ohms, the respective xalues of R-i necessary for maximum 

 received current strength, range from approximately 624 to 935 ohms. 

 If, then, resistance be added in the proper amounts at the points desig- 

 nated (2), Fig. 1, the received current will be increased thereby and at 

 the same time, the line current will be reduced. If the line circuit re- 

 sistance is approximately 1650 ohms or more the amount of resistance 

 needed at points (2) to make the received current maximum, will be 

 sufficient to reduce the line current to .070 ampere or less. This is 

 illustrated by the three upper curves in Fig. 2. The lower broken 

 curve, designated /o, represents the received current which will be ob- 

 tained with no regulating resistance in the circuit at either point. It 

 w'ill be seen that this curve passes below curve io at a point correspond- 

 ing to a line resistance of 1650 ohms. With approximately that value 

 of line resistance and no regulating resistance, the line current is ap- 

 proximately .086 ampere and the received current is .0171 ampere. If 

 approximately 410 ohms be added at points (2) the line current will be 

 reduced to .070 ampere and the received current will remain at .0168 

 ampere. Curve /max. shows the maximum received current which 

 can be realized by adding correct amounts of resistance at points (2). 

 The upper cur\e touches curve 12 at a point corresponding to a line 

 resistance of 1850 ohms. That is, with a line resistance of this value, 

 the regulating resistance required to reduce the line current to .070 

 ampere is just sufficient to bring the received current up to the maxi- 

 mum. For lines of this resistance or greater, the line current can be 

 reduced to .070 ampere or less and at the same time the received cur- 

 rent is increased. It will be seen from Fig. 2, that as compared to 

 locations (1) and (3) for the regulating resistance, the advantage of 

 location (2) from a steady-state received current standpoint, becomes 

 greater with lines of low resistance and amounts to 32.3% and 60.1% 

 respectively, with a line of 500 ohms resistance. On the other hand, the 

 increase in received current due to arrangement (2), as compared to 

 the condition of no regulating resistance, becomes greater with lines 

 of higher resistance, as shown by the divergence of the ii and iniax. 

 curves. Fig. 2. 



With line resistances in the lower range, the amount of regulating 

 resistance needed to make the recei\"ed current maximum will not be 

 enough to bring the line current down to the desired value of .070 am- 

 pere. For example, with a line of 500 ohms resistance, the 500 ohm 



