Following this development, the use of hard drawn copper wire became the standard for long telephone lines, first in the Bell Telephone System, and later the world over, greatly ex tending the range of transmission. In 1892, New York was placed in telephonic communi cation with Chicago. The wire employed for this circuit, which first realized the conunercial 1,000-mile talk, was of hard drawn copper No. 8 B. W. G. in size (0.165 inches in diameter). The pair of wires required to connect Chicago with New York weighed about 870,000 pounds — a full load for a 22 car freight train.
Before the local service wires in cities had reached such numbers as to necessitate ing them underground they had been strung on poles and roof tops. This was objectionable on account of the corrosion of the iron wires by chimney gases, wires breaking when loaded vvith sleet, the cost of roof-repairing, the sightliness of the construction' and the lack of room for more wires. It was, how ever, the only possible way known at that period. The overhead method was soon outgrown. Some streets in the larger cities had become black with wires. Poles had risen to 50 feet in height, then 60, 70 and 80. Finally, the highest of all pole lines was built along West street, New York City, using. 90 foot poles of Norway pine, and carrying 30 crossarrns and 300 wires. (Fig. 45). This condition of wire crowding. was overcome by the development of cables, later described.
Telephone Grounded Circuit Dif ficulties.— When the ground was used for the return circuit, not cmly was the talking distance limited by the confus ing effect of all sorts of disturbing cur rents from the atmosphere and from neighboring telegraph wires, but also, when a second telephone wire was strung alongside • of the first, even though perfectly insulated from it, con versation carried on over one of these wires could be heard plainly on the other.
The Balanced Metallic Circuit. Open Wire Lines.— In 1883, it was discovered that the bad effects of these disturbing currents could be greatly lessened by severing the earth connec tion from the ends of each wire, and substituting a second wire for the re turn circuit through the earth. The new arrangement was known as the °metallic circuit)) For the reason that the °metallic circuit)) principle involved rebuilding practically the entire plant, telephone engineers sought less costly methods of reducing the electrical dis turbances.
Between 1885 and 1892, large numbers of so-called anti-induction devices were proposed and investigated. None of them proved sufficiently successful to warrant adoption. Those that killed in duction killed transmission as well. A system using a common return , wire for a number of telephone circuits was used to a considerable extent. 'It re duced the disturbances due to ground notentials but was not effective atrainst induced currents. The reconstruction of
the telephone plant on the metallic cuits basis was carried out largely be tween 1890 and 1900.
To aid further in rendering metallic cir cuits free from induction from power circuits and from crosstalk, the wires were transposed (i.e., interchanged in position on the cross arm) at periodic intervals of space.
The first transposition scheme was worked cut in 1886.
Fig. 46 shows a typical transposition dia gram for the 10 wires carried on one crossarm of a line. On this diagram the vertical lines represent poles 1,300 feet apart, that is, about every 10th pole. The interchange in position of the wires is indicated by the crossing of the horizontal lines which represent the wires. At the points indicated by small circles, special types of transpositions are placed to admit of phantom circuit working (described below). To ensure quiet circuits, the apparatus used with metallic circuits has to be carefully balanced when arranged for talking. Wherever a ground connection is used, as in some forms of signalling, it has to be placed at a neutral point in the telephone circuit.
Balance in the line circuits is attained by uniformity in the wire used and by suitable arrangements in its installation and main simultaneously over the -two wires of a circuit, they produce no sound in the telephones sociated with that circuit. Fig. 47 shows how the phantom current is equally divided, by means of connections to the middle points of repeating coil windings, so that it traverses tenance. Any slight imperfection in the balance of the line conductors in phantomed circuits manifests itself by crosstalk be tween the phantom and its constituent cir cuits. While the metallic circuit, trans posed according to the methods first de vised by the Bell engineers, eliminated the out 'side disturbances that existed at the time, this was not the state of affairs for long. Power ful electric light and power circuits began to spring up and methods which had formerly been successful in protecting the telephone from disturbing currents were not effective against these new high powers. Years and years of work have been devoted to safeguarding the ' telephone circuit from these disturbances, and each new success which the telephone engineer has accomplished in this direction has been fol lowed by a further advance in high potential and high current on the part of the power cir cuits. It has been said with truth that if these high power circuits had been discovered and in use before the telephone was invented, the re sults obtained from the first telephone lines would have been so utterly impracticable ttiat it is hard to think of any one being rash enough to regard the telephone as having any commercial value.