Line Current Regulation in Bridge Polar Duplex Telegraph Circuits

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 maximum 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 received 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 difficulty is encountered in operating a duplex telegraph circuit with the regulating resistances iti the line, a very decided improvement is obtained by transferring these resistances to the bridge arms.