Publication: Scientific American
New York, NY, United States
The Insulation of Telegraph Wires in Cities.
Glass, when placed in the shade, becomes completely coated with a thin film of water whenever the moisture contained in the atmosphere amounts to above 40 per cent of saturation. During the rain the atmosphere sometimes reaches the point of complete saturation, or 100 per cent. When this is the case, any article of glass, even if exposed to the atmosphere alone, and not to the direct action of the rain, is soon completely covered with moisture, and under these circumstances its surface becomes a conductor of electricity.
The atmosphere of all large cities is heavily charged with soot, smoke, and ammoniacal salts, arising from combustion, and these, being taken up by the particles of falling rain and moisture, increases the conducting power of the latter to an enormous extent. Careful experiments made in Manchester, England, where the atmosphere is very impure, showed that the conducting power of the rain water which fell in that city was more than 300 times that of distilled or absolutely pure water. Speaking of this subject, Latimer Clark says: "Pure water offers a very high resistance, but if it contains any acids or saline matters in solution, the resistance is much smaller; hence that clear rain in the country does not greatly injure the working of a line, but in towns, where the atmosphere is less pure, the insulation often becomes very imperfect in wet weather."
The comparative insulation of wires in the city and country, under otherwise similar conditions, may be seen by the following actual measurements, taken at the New York office of the Western Union Company: No 1 wire east showed a mileage insulation, between 145 Broadway and Harlem river, of 66,000 ohms, while from Harlem river to New Haven Conn., the same wire gave 282,000 ohms per mile. No 3 east, to Harlem, gave 53,500 per mile; Harlem to Hartford, Conn., 218,000. The insulation in the country exceeded that in the city in the proportion of more than 4 to 1.
The European telegraphic engineers have endeavored to surmount this difficulty by changing the insulators at short intervals, as their surfaces become smoked and dirty. This, however, is but a partial remedy, as the trouble arises as much from the great conductivity of rain water, under the conditions referred to, as it does from dirt upon the surface of the insulators. They have also largely resorted to expedient of running the wires underground, a method involving great expense, and yet or rather questionable benefit, as far as immunity from interruption is concerned. Considerable embarrassment is also occasioned by inductive action, when underground wires are employed, especially in working automatic or printing instruments.
It is to an American inventor that the credit is due of being the first to discover a practical and effectual means of insulating wires in cities; and equal credit should be accorded to the American telegraphic superintendent who had the boldness to put the plan into practice on a large scale, with the most successful results-We refer to the magnificent lines build by General Anson Stager, of the Western Union Company, in the principal western cities, which are considered by competent judges to be, perhaps, the finest examples of telegraphic construction in the world.
The height of the city poles above the ground is sixty-five feet. They carry fifty No. 9 wires, arranged upon nine cross arms, and insulated with the Brooks insulator. A test of these lines in rain, after two years exposure, shows the insulation, within eight miles from the office, to be so high as to be beyond the range of measurement of either the Siemens universal galvanometer or the Varley differential — the instrument usually employed for these tests. These lines, as specimens of telegraphic engineering, are equally creditable in a mechanical point of view. The massive spars, ranged with mathematical accuracy for miles along the straight and level streets of Chicago, instead of detracting from the appearance of the thoroughfares, are a positive ornament to them. The ordinary sized poles are twenty-one feet in height, and fitted with similar insulation. These are used on the Central Pacific Railway line, the Michigan Central, and the Philadelphia and Reading Railway line. The latter, by the way, is a very good specimen of substantial construction, eight wires being carried upon two cross arms, and not high enough from the ground to strain the poles too much upon the sharp curves which abound upon that road.