Telephone circuit unbalances determination of magnitude and location

The maintenance of telephone circuits in a high state of efficiency with respect to balance is important since unbalances contribute to crosstalk between telephone circuits and to noise when such circuits are involved in inductive exposures. Methods and apparatus are described which afford operating telephone companies means for maintaining their circuits in the condition of minimum practicable unbalance. Different types of unbalance are discussed, also their effects under different conditions of energization of the unbalanced circuit and neighboring conductors. Methods are described for determining: (1) The general condition of circuits with respect to balance by crosstalk measurements from their terminals, (2) The approximate location of unbalances along a line by measurements over a range of frequencies with a bridge at one end of the line, and (3) The final location of unbalances by field measurements with an unbalance detector which may be operated by a lineman and does not usually require interruption of telephone service, except momentarily. Toll circuit office unbalances are briefly discussed and a special bridge for detecting, locating and measuring the unbalances of composite sets is described. In an appendix is given a mathematical treatment of the bridge method for locating unbalances and a discussion of the necessity of terminating the circuits involved in the tests in their characteristic line impedances.