Inductive disturbances in telephone line

The inductive action of one telephone line on another, producing what is commonly known as cross-talk, presents an important problem to the telephone engineer. The induction is both electromagnetic and electrostatic. A variable current passing through any circuit is accompanied by a variable magnetic field, which will produce electric currents in neighboring circuits. In addition to this there exists also an electrostatic effect, a charge on one conductor will induce charges on all neighboring conductors, and any variation in the charge will produce an electric current. We thus have two distinct phenomena acting simultaneously, both causing trouble on telephone lines. It is to be noticed, however, that the two effects act in opposite directions. The establishment of a current in one circuit is accompanied by an induced current in any neighboring circuit in the opposite direction, while when the current in the inducing circuit is decreasing, the induced current will be in the same direction. In the case of electrostatic induction the conditions are reversed. When both forms of induction act together we obtain a resultant effect which depends on the various electrical constants of the lines and their relative importance. It would appear therefore quite evident that to obtain a knowledge of the effect due to induction, we must first be able to ascertain the relative importance of one form of induction as compared with the other, and to determine the factors which enter in fixing the magnitude of each. A thorough knowledge of these factors may be of considerable assistance in improving the conditions of telephone lines, or at least it may suggest some methods for overcoming induction effects which are a source of considerable annoyance to users of telephones.