Telephone Circuits with Zero Mutual Induction

The paper deals with the reduction of inductive interference in telephone circuits. Various relative positions of two or more circuits, in which the mutual inductance is zero, and the mutual capacitance unbalance is approximately zero, are discussed. The most important case is that when the two wires of one circuit occupy opposite ends of one diagonal of a square, and the other circuit, the ends of the other diagonal. A phantom formed on circuits so related is nearly immune from external induction. Several forms of construction embodying this arrangement, and built largely with standard parts, are illustrated. In these forms, about one third of the inductive relations of each physical circuit are with circuits in a position for approximately zero mutual inductance. The form of phantom transposition used automatically balances the circuits not so related. Calculations and tentative designs are presented to show that the use of these forms of construction will give greatest refinement of balance against induction from power circuits, and possibly also against cross-talk, increased flexibility in coordinating with the variations in exposure to power circuits, a simplification of the transpositions system, fewer transposition poles and transpositions, and when desired, the realization of a part of these advantages with the lead compressed into less than the normal space. The improvement is greatest in the phantoms. Adverse factors are the increased sensibility to cross-talk due to variations in sag, and the slightly increased capacitance of the phantoms.