Publication: The Telegrapher
New York, NY, United States
The Insulation of Telegraph Wires in Cities.
THE difficulties attending the working of telegraph lines in large cities, in rainy and damp weather, were a subject of common remark among practical telegraph men for many years before the philosophy of the matter was at all understood. The operators of the House printing lines, which, at the time of their construction in 1848, were built and insulated with much more care than any of the lines previously existing in this country, were often surprised to find that in long continued rains, when Poughkeepsie could work to Utica, and even to Buffalo, it was almost impossible to communicate with the New York office. The same phenomena were manifested to an equal extent upon the wires running from New York to Boston. The most generally accepted explanation of these facts, we remember, was that the atmosphere of New York City was possessed of unusual conducting qualities, it being universally supposed in those days that the current of a telegraph line, in damp, weather, escaped from the wire directly into the adjacent moist atmosphere, and through that to the ground. The part played by the insulators in the transaction was at that time little known or suspected.
It was not until the introduction, within a very few years, of instruments for the detection and measurement of delicate electric currents, and the means which was thus afforded of ascertaining with absolute accuracy the capacity of different substances for conducting electricity, that the true cause of these troubles began to be understood, and means of obviating them sought for with any prospect of success.
The first step towards the solution of the problem was the discovery of the hygroscopic qualities of glass, which, from almost the first introduction of the telegraph, had been exclusively employed to effect the necessary insulation of the wires. 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 the saturation. During 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, increase 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 it is 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 surface became 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 the expedient of running the wires underground - a method involving, great expense, and yet of 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 telegraph superintendent who had the boldness to put the plan into practice on a large scale, and with the most successful results - we refer to the magnificent lines built 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.
Fig. 1 will serve to give an idea of the arrangement of these lines. It is a drawing of a pole on one of the principal routes in Chicago. The height of this pole above the ground is sixty-five feet. It carries 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. In order to afford an adequate idea of the size and substantial construction of these lines, we give, in Figs. 2, 3 and 4, representations of some ordinary sized poles, twenty-one feet in height, and fitted with similar insulation. These are drawn to the same scale as the Chicago pole, and represent the relative sizes accurately. Fig. 2 represents the Central Pacific Railway line, Fig. 3 the Michigan Central, and Fig. 4 the Philadelphia and Reading Railroad 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.
* "Electrical Measurement," page 4.