Article by David Brooks on capacity of telegraph lines

ON THE WORKING CAPACITY OF TELEGRAPH

LINES.

BY DAVID BROOKS.

[Concluded.]

ONE of the most prominent features in the construction of the English telegraph lines, and the first to attract the attention of the American telegrapher upon visiting that country, is the size and weight of the wires used as conductors. The wires used for the longer circuits are Nos. 3 and 4 Birmingham gauge, which are larger than Nos. 2 and 3 American gauge. On each side of the railways are a line of posts, carrying from eight to ten wires, of which generally six out of eight are of the larger sizes, and underneath these are two or more No. 8 wires, for the shorter and less important circuits. Although the latter are equal to the largest size employed in America, they appear very small in comparison with the large wires above. To each post is attached a lightning conductor or "earth wire." The principal object of this arrangement is to prevent " cross currents" from one wire to another, or what the English telegrapher terms "confusion of signals," by cutting off the currents passing from one wire to another and conducting them into the ground. The insulator employed there is of brown earthenware, with a salt glaze. It is more hygroscopic than glass, and inferior to the latter as an insulating material. The principal involved in this arrangement is that of working an uninsulated wire through water, by simply making the resistance of the former very small in comparison with that of the latter. Their theory is that the insulator is benefited by being washed by the rain. If this theory is correct the advantages realized ought to be very great, for it rains there almost every day. In order to enable the posts to sustain the weight of these heavy wires it is necessary to employ at the angles and curves contrivances known as ".struts," "stays," and "double frames." In this manner the English telegraphers endeavor to overcome the effects of leakage at the insulators by making the latter not absolutely small, but relatively small in comparison with the conducting capacity of the wires.

A careful galvanometrical test of the English insulators, in comparison with the paraffin insulator, which was made in London, showed a difference at the end of one month in favor of the American insulator in the proportion of 60.000 to 1! Tests made in this city, at the end of one year, show a much greater disparity, being in fact no less than 200,000 to 1. Now the same causes that impair the value of the English insulators will in time produce the same effect upon these. The resistance of five of these insulators was reduced by artificial means, that is, they were placed directly in the current of smoke from a chimney for a period of six months, until they gave under test, during rain, an average deflection of 60,000 degrees. The English insulators, which had been exposed merely to the ordinary atmosphere influences, when tested at the same time, showed a greater leakage.

During the continuance of the rain the paraffin insulators were cleaned with a brush dipped in liquid paraffin. The first insulator gave under test a deflection of 60,000 degrees. We set the person at work cleaning it, and in five minutes stopped him and tested again. The deflection was reduced to zero. Another insulator, time four minutes, gives the same result, as did, also the third with three minutes, and the fourth with two minutes. But one minute was allowed for cleaning the fifth insulator. This brought the leakage down from 50,000 to 1,500 degrees. One minute more of cleaning brought the index of the galvanometer to zero, the same as the others. We can take any of these insulators, which have deteriorated by exposure to the weather, and completely restore their insulating qualities at the same trifling expense.

With a knowledge of these facts, does it not occur to the reader that the means employed by the English telegraphers to secure a working margin between their insulators and conductors is needlessly expensive. The postmaster general, when called on by Parliament recently to account for the frequent interruptions and delays to the telegraphic service, gave but one reason, and that was "defective insulation." This is but the inevitable result of attempting to carry out in practice a theory that is radically defective.

The writer was shown, at the Government Telegraph Works in Paris, in 1867, a lot of wire in bundles, piled eight feet high, and covering, as nearly as could be judged, about an acre and a half of ground. It may have been less, but was quite as like to have been more. This was condemned wire, or rather wire which had been taken down from the telegraph lines and replaced by a larger size. The greater part of this condemned wire was of four millimetres diameter, a size not varying greatly from that known in this country as No. 8. It was in excellent condition, being nearly as bright as when new, and of the best charcoal iron. Its quality was superior to that of any wire which has ever been used in this country. This wire had been replaced by a size of five millimetres diameter, equivalent to about No. 3 of the American gauge. The French Government had incurred this great expense in order to improve the working condition of their lines, but the results failed to answer their expectations. The effective margin, it is true, was considerably increased, but the escape from the insulators was still a serious evil. They then turned their attention to the removal of the latter difficulty, and by changing the insulation they increased the working margin immensely. They were by these means enabled to work direct to Bordeaux, Marseilles and Havre, and through circuits nearly a thousand miles in length, in rain, at full speed, with the printing instrument. These circuits had previously been worked by the aid of repeaters, by the Morse system, and at a great disadvantage. A letter from one of the Inspecteurs states that they now work two wires from Paris to Berlin with the Hughes printer, at full speed, during rainy weather. In fine weather it has been observed that there is an interference arising from inductive action between these two wires, which are upon the same poles, and that therefore they cannot work both wires at full speed at the same time - a singular effect, arising from the exceptionally high insulation.

The capacity of the Hughes instrument for speed is determined by the number of times any given word of average length - for instance, "Paris" - can be printed on short circuit in one minute.

The insulators which were thrown away when the above mentioned change was effected, were made of the best French china, the most perfect material of the kind which has ever been discovered. These insulators were beautifully glazed, and were in possession of all the advantages which could be derived from being "washed in the rain."

The French and Prussian telegraphic engineers have learned from experience that an insulator, to be effective in rain, must have a well protected drip or shed, and this portion of the insulator must be kept clean. The credit of designing the first insulator upon correct scientific principles is due to Genl. Chauvin, Director of Prussian Telegraphs. It has great length and small diameter. The paraffin insulator has still greater length, and its resistance is proportionately higher. The latter is also much more easily cleaned and kept in order. If there is any design at all in the arrangement of the English insulator, it apparently consists in rendering it practically impossible to clean it. They are fastened to the cross-arm with bolts and screw-nuts, which become rusted by exposure, so that it is almost impossible to loosen them. Without removing the insulator from the arm it is impossible to get at the inner surface in order to clean it, and when cleaned its surface will retain more moisture than glass. If the English postmaster general has come to the definite conclusion that their system is "defective," there is some probability of an improvement before long. The English notions of insulation are perhaps not more peculiar than some other features of their telegraphic system, which they have clung to for years with a pertinacity no less obstinate than amusing. The era of "needle instruments" and "sand batteries," in that country, is even now but just passing away.

The intelligent reader will at once understand, from what has been said, that all efforts to improve the present defective condition of our telegraph lines, and to increase the effective working margin, must be made either in the direction of improving the insulation of the conductor or of reducing its resistance. The problem, therefore, resolves itself into the inquiry: Which is the most economical and convenient means of attaining the desired result? A brief consideration of the trouble and expense necessarily involved in replacing the present conductors with others of larger capacity, compared with that of substituting the most perfect insulators known for the inefficient ones in general use, is sufficient to answer this question. The improvement that may be effected in the condition of a line, by thus perfecting its insulation, is simply equivalent to the difference between the working of any circuit in fine and in damp and rainy weather.

It has been ascertained, beyond a doubt, that the escape from a telegraph line is due solely to conduction of the insulators, and of foreign bodies in contact with the wire. It is found that the opinion which prevailed for many years, that a considerable portion of this leakage was due to the conduction of damp air, is not supported by fact. The amount of current lost in this way is far too small to be detected by the most sensitive apparatus employed in modern electrical researches.