lines; then he installed a home-made switching device in his office, and for a signaling device employed a single-stroke bell operating on a gravity-battery current. Then he devised a method of preventing eavesdropping that in May, 1878, was declared 'a great improvement on any other telephone system now in use.' So satisfactory did these instruments prove that other lines were similarly equipped and soon a mutual telephone exchange system was in full operation, that later on formed the nucleus of the commercial exchange opened in Bridgeport by Mr. Doolittle in 1878.
In the fall of 1877 he planned and built a private telephone exchange system for the Ansonia Brass and Copper Company, of Ansonia, that, while in no sense a commercial exchange system, was indirectly of incalculable benefit to the growth and prosperity of the entire commercial telephone industry. For in planning this system Mr. Doolittle decided to use circuits of copper instead of iron, and, after many experiments, produced a hard drawn copper wire of his own adaptation, the drawing of which he supervised in the mills of that company.
Prior to that time it had been found impossible to use copper wire on pole line circuits, as its extreme ductility proved a source of continued elongation in all spans of any length and where the strain was constant. By the Doolittle process the tensile strength of the wire was greatly increased, its elongation reduced to about one per cent., and there was no appreciable change in its conductivity. Yet it required ten years of costly experience with iron wire circuits before the telephone interests fully comprehended the inestimable value of this improvement. In September, 1880, at a conference of telephone men, the representative of a very large wire-drawing mill stated that "copper wire has ceased to be discussed for telephone line use. It is too soft and elongates too readily under exposure. The suitable wire must be tough pure iron, well manipulated to secure flexibility and toughness."
Referring to this pioneer work, Mr. Doolittle wrote: