Motion of Telephone Wires in Wind

N T I L recent years, most of the important open wire toll circuits of the Bell System had the two wires of a pair spaced 12 inches apart. This wide spacing, with the consequent high mutual inductance between the several pairs on a pole line, limited the use of the lines for multiplex transmission with high frequency or "carrier" currents. A reduction in the separation of the wires of a pair with the retention of the present center-to-center spacing of the pairs was one of the measures which offered the opportunity of increasing the message carrying capacity of a pole line. The controlling factor in limiting such a reduction in spacing was the hazard of the wires of a pair swinging together in the wind thus interrupting or impairing the transmitted messages. About two years ago the 12-inch spacing was reduced to 8 inches in some cases. This was considered to be as great a change as could be safely taken from a mechanical point of view, based on the available data. These data consisted in part of experiments made on an experimental line and in part of an analysis of the performance of certain working wires in the telephone plant which, for various reasons, had been installed on a close spaced basis. It was realized that if the wires of pairs could be placed even closer together, materially lower crosstalk between the circuits would result, thus increasing the circuit capacity of open wire lines, and therefore effecting economies. Accordingly, a comprehensive investigation of the wire spacing problem was begun.