Popular Science Monthly/Volume 24/February 1884/Under-Ground Wires
THE first telegraph line constructed in this country, from Baltimore to Washington, in 1843, was intended to be laid underground, and the first nine miles was so laid. Four copper wires were each wound with cotton, soaked in shellac, and the whole drawn into a lead tube. This tube was laid in a trench by the side of the rail-road. Hardly was the section completed, however, when water found its way into the joints, destroying the insulation, and the conductors failed. They were accordingly replaced by wires strung on poles, and the rest of the line was constructed in this way.
In England a very similar line was built, along the line of the Great Western Railway, for a distance of thirteen miles out from the city of London. This line failed in exactly the same way as the American lines, and the pipes were dug up and placed on short posts six inches above the ground. They were, however, soon replaced by pole lines.
At various places on the Continent similar experiments were tried, and everywhere with the same results. Thus it happened that, though the first idea of telegraph engineers the world over was to run electric wires under-ground, they were everywhere obliged to string the wires on poles. In England and on the Continent there has always been a strong desire to have a part, at least, of the electric wires under-ground. In the cities, pole lines have been considered objectionable, because they disfigure the streets. Between cities, under-ground lines have been desired, because of their great safety in case of invasion, great secrecy, and reliability in case of storms.
The introduction of gutta-percha, in 1846, accordingly gave a new impetus to under-ground construction, and, though it took years of experimenting and millions of dollars, and though system after system failed in England, Germany, and the rest of Europe, there exists to-day a successful and durable system of under-ground telegraph wires connecting together the principal cities of the German Empire, besides many other under-ground lines in various parts of Europe. Many of the European cities have the telegraph lines carried from the center of the city to the outskirts, under-ground; and, in Paris, not only all of the telegraph lines, but those for electric lights, telephones, and the various other private and municipal lines, are carried in the sewers under the streets of the city.
It must be remembered, however, that these various systems have cost from ten to twenty times as much as similar overhead lines; that, for every mile of under-ground wire, there are many miles on poles; and that in Paris, which is the only city in the world having a complete under-ground system, there are unusual facilities for the running of wires, as sewers large enough to walk about in extend even under the less important streets of the city. Moreover, it has been found that, for delicate and quick-working apparatus, such as automatic telegraphs, polarized relays, and, above all, the telephone, long underground lines are far less efficient than pole lines. There are two reasons, apart from the difficulty of securing good insulation, why these long under-ground lines are comparatively inefficient:
1. If an electric conductor be brought near to a large mass of conducting matter, as is a wire when it is taken down from a pole and buried in the earth, there appears in the current the phenomenon of retardation, by which each signal, instead of being sharp and distinct, is partly kept back, so that it overlaps and mingles with the next; the result is to limit the speed of working of the apparatus; or if, like the telephone, it be an apparatus in which the currents are necessarily extremely frequent, to confuse and destroy the signals altogether. With ordinary Morse telegraphic apparatus, this is not very troublesome on under-ground lines a hundred miles long. With delicate relays, and more especially with quick working printing telegraphs, or automatic telegraphs, such lines are very troublesome; and, with telephones, the retardation is a very troublesome matter on under-ground lines ten miles long.
2. The second difficulty is called induction, and is noticed when two or more wires are run side by side and near together, as they necessarily are in an under-ground cable.
If the signals on one wire of such a cable be sharp and quick, they causesignals on all of the neighboring wires, and this too, though the insulation may be absolutely perfect; indeed, above a certain point, the more perfect the insulation the greater the induction. The result of this phenomenon is, that messages sent over one wire are liable to be received on all of the other wires, and, in the case of the telephone, this phenomenon is noticeable on cables one thousand feet long, and on a cable one mile long the parties on one wire can easily understand what those on the other wires are saying. For any other instrument, however, the interference only becomes annoying on much longer lines. Steady currents, like those used with electric lights, are, of course, not affected either by retardation or induction.
In our own country there is little doubt that the proper method of constructing electrical wires between cities is, to string them on poles in mid-air. A brief review of some of the European systems that have been constructed will convince us of this. Between the years 1847 and 1850 a system of cables, containing 2,648 miles of wire, was laid under-ground to connect Berlin with the other principal cities of Prussia. Gutta-percha-covered wires were drawn into lead tubes, which were then buried in trenches two feet deep. The cost of this system was at least ten times that of well-constructed overhead lines. By 1850 the earliest of these lines had failed, and by 1853 the entire system was replaced by pole lines. In 1852 cable was laid in Russia, between St. Petersburg and Moscow; this worked a few years and then failed. Between 1846 and 1852 many miles of somewhat similar cables were laid in France, but, excepting those laid in the sewers of Paris, they universally failed.
In 1854 quite a number of lead-covered cables were laid in Denmark, but these were soon obliged to be abandoned in favor of over-head lines. In 1853 the Telegraph Company of England laid down a cable of ten gutta-percha-covered wires, in wooden troughs, along the high-road between London and Manchester, a distance of two hundred miles. Although neither expense nor pains were spared in the construction of this line, the cost being comparable with that of the Prussian system, two years had not elapsed before some of the wires ceased to work, and, though these were replaced and workmen kept constantly busy on the line, at the end of seven years the line was wholly abandoned in favor of overhead wires.
During the same year the Electric Telegraph Company laid down a somewhat similar system between London, Manchester, and Liverpool, though iron and earthenware pipes were substituted for the wooden troughs. Some of these lines began to fail almost as soon as completed, while others were, by constant repairing and attention, kept working for nearly ten years, when the whole was finally abandoned and overhead lines put up.
The great trouble with all of these systems, whether in England or on the Continent, was due to water, which found its way to the conductors, and of course destroyed the insulation. It was difficult to handle the wires without abrading the gutta-percha; and, when safely laid, the gutta-percha was attacked by coal-gas, vegetable growths, and the constituents of the soil. During this time many other shorter lines were constructed, but invariably with the same results.
In 1855 the French government, having failed in their attempt to use gutta-percha wires, laid down a large number of bare wires in a trench filled in with bituminous compounds. The details of this work were very carefully carried out, and the experiment is of interest because similar plans are constantly being proposed to-day. This system, costing from eight to ten times that of a thoroughly built pole line, never worked satisfactorily, and soon had to be abandoned. In 1858 the administration decided to return to gutta-percha-covered cables laid in lead tubes. The reason of this was, that some of these cables laid in the sewers of Paris, in 1846, were still in good condition. Many miles of this cable were laid, some with the lead pipe laid directly in the earth, some with it drawn again into iron pipes, and some carried through the sewers of the principal cities. Those cables laid directly in the earth soon failed, but those in iron pipes and the sewers continued to work, and from this grew the system now used in Paris. Up to 1870 the above-described attempts, as well as many others (not recounted), had proved a series of complete failures. Since that, however, several lines have been built in England that have continued to work successfully; and in Germany successful under-ground cables have been laid down connecting together all of the principal cities of the empire. The present complete system, as used between Liverpool and Manchester, was constructed as follows: Iron or stoneware pipes were laid from one to two feet below the level of the road-side with flush-boxes coming to the surface every two hundred yards. Into these was drawn a cable of gutta-percha-covered wires. The joints were carefully made in the pipes, and they were smoothed inside to prevent any possible abrading of the cable. The route was especially selected through a low and marshy section of country, so that the pipes were almost constantly filled with water—this being the best possible condition for the preservation of the gutta-percha. The present European system dates from 1875. The cable is similar to that used for submarine purposes. It consists of seven copper wires, each coated with two layers of gutta-percha and two of Chatterton's compound, and the whole covered with an armor of galvanized-iron wires. This cable is laid in a trench by the road-side, and comes to the surface only inside the telegraph-offices in the cities. Its cost was nearly twenty times the cost of a well-built pole line.
Although both the English and the German systems are successfully working lines of telegraph, they are far less efficient than pole lines of the same length. The speed of working even the ordinary instruments is limited; serious trouble appears in attempting to use fast-working machines, or automatic senders, and the use of the telephone is impossible.
I think these facts have sufficiently demonstrated that for long lines of telegraph, stretching from city to city, here in America, pole lines, which can be cheaply built, easily repaired, and where the wires can be removed from the retarding influence of the earth and the inductive influences on each other, are decidedly superior to underground lines.
Within our large cities the problem presented is somewhat different. During the last few years the number of electric wires has rapidly increased, especially since the introduction of the telephone and electric light, and the probability is that the next few years will show a further large increase. If these wires are run on poles, they not only disfigure the streets, but seriously interfere with the operations of firemen in case of fire, as we have repeatedly seen during the last few years. A cobweb of wires running over the house-tops requires the linemen to continually tramp through the houses and over the roofs, causing annoyance to the tenants and damage to the buildings. Moreover, wires fixed to house-tops are subject to removal at the whim of the owner, and they have to be continually removed from building to building as the good-will of each owner is exhausted.
In almost all of the large cities the question is now being asked, Why can not all of these wires be buried along with the gas and water pipes under the streets? In answer, I propose to describe briefly what has been done in this direction in European cities, then to look at some experiments lately made in this country, and thus to show how far such a plan is and how far it is not practicable.
In Paris, all the wires are carried in the sewers under-ground.
In London, the telegraph wires are carried from the central office to many of the branch offices and to the railways leading out of the city under-ground.
In Vienna, Prague, Brünn, Munich, Augsburg, Nuremberg, and many other cities, the telegraph-wires are carried under-ground by armored cables to the outside of the city.
In the German cities we have seen that many of the telegraph-wires are carried under-ground from the center of the city to connect with cables running to other cities.
Telephone wires, electric-light wires, and a large majority of telegraph wires in European cities are, however, as in America, carried over house-tops or on poles.
The cable most generally made in Paris consists of seven gutta-percha-covered wires laid into a cable covered with tarred hemp and drawn into a lead pipe; this pipe is fastened by hooks to the side-wall of the sewer. The cables are thus easy of access, and any new cables may be added as required without disturbing those already in use. In some of the newer cables wires covered with cotton soaked in paraffine are used instead of gutta-percha-covered wires. The distances within this city are so short that neither induction nor retardation has to be considered in the telegraph wires. Electric-lighting wires, we have seen, are not affected. The telephone wires are in Paris protected from these evils by an extremely simple though expensive device. Instead of a single wire for each circuit, two wires twisted together are used, the current going out over one and returning over the other. Such a device is called a "metallic circuit." Any outside disturbing circuit tends to induce, in the two wires of the metallic circuit, equal and opposite currents, which neutralize and disappear. In such an arrangement, too, there is a minimum of retardation.
There are several thousand miles of wire in the sewers of Paris, and the cost of the gutta-percha-covered cables is about $140 per mile of wire, or about five times the cost of a pole line to do the same work. As telephone cables require two wires for each circuit, this estimate would have to be doubled. The paraffined cables are, however, considerably cheaper, though their durability has not yet been proved. The cost for repairs is very small, and some cables have not been touched for twenty years. In any other city than Paris, the above figures would be very greatly increased by the cost of under-ground piping and chambers to contain the cables.
It is thus demonstrated that it is technically possible to place all of the wires in a city under-ground. It is also demonstrated that the cost, even when a large number of wires run side by side, is enormously increased. For many purposes, as telephony or electric lighting, a considerable number of wires start out from a central office together, but continually bifurcate until single wires run to the houses of the subscribers. The cost of one wire by itself is vastly larger than where many are run together, the cost of the pipe and for laying being not much greater for fifty wires than for one, and the cost of single wire cables being greater per mile of wire than multiple wire cables, so that the expense of putting such a system as one of our telephone exchanges entirely under-ground would place the cost of the instruments entirely out of reach of the subscribers. If telephones were required in every house, as are gas and water, such a system might be practicable, but at present that is not likely to be the case.
The American Bell Telephone Company has recently constructed two short lines of under-ground wires in the business section of Boston, and these give us excellent data from which to judge of the extent of technical practicability and the expense of putting all wires under-ground. We have seen that in Paris the retardation and induction are both obviated by the use of double and twisted wires in metallic circuit. It is necessary that all of the wires be in metallic circuit, for, if a metallic circuit be connected to a single-line circuit, the disturbances are not removed. If a subscriber in one city wishes to talk with a subscriber in a neighboring city, both cities must have metallic-circuit systems and metallic circuits between the two cities. As the two lines constructed in Boston are short, only about one quarter of a mile each, it was deemed best to use single-line circuits, hoping that the induction and retardation on so short lines would not be serious.
The system is constructed as follows: Eight wrought-iron pipes, three inches in diameter, are laid side by side in two rows, about four feet below the surface. At each street corner is built a brick chamber, large enough to admit a man, and with a cover flush with the street. The cables, of which several kinds are in use, run out from the basement of the central office through these pipes and up the side of buildings to roofs, from which they spread out to the subscribers by means of ordinary overhead lines.
Conversation over these lines is not so easily carried on as by means of overhead wires, and it is frequently possible to overhear other conversation. This prohibits further extension of the single-wire system under-ground, for technical reasons. The cost of the piping and chambers is in round numbers $50,000 per mile, and these pipes are intended to accommodate one thousand wires. The cost of the cables is from $60 to $150 per mile for each circuit, according to the kind of cable used.
In round numbers we may estimate the total cost for one thousand wires at $150,000 per mile, or $150 per mile per circuit. The cost of piping and chambers would be nearly as great for one hundred circuits as for one thousand, as the cost of chambers and the labor of excavating and filling would be the same; so that the cost for one hundred wires may be estimated at $50,000 per mile, or $500 per mile per conductor. The cost per conductor thus increases enormously as the number of conductors diminishes, so that it would be clearly impossible to follow out the wires of an exchange system in all of their bifurcations.
It may be argued that cheaper methods of laying wires may be devised; but the experience of forty years has led continually to more and more expensive systems. If, then, the present method of running wires overhead is objectionable, and the expense of running them under-ground is so great as to put the cost of telephones, electric lights, and other electrical appliances out of the reach of would-be users, how are the wires to be run?
It seems to the writer that much of the inconvenience may be obviated, and without greatly increasing the expense, by adopting the following plan: From each telephone exchange, electric-lighting station, or other center of electric wires, run overhead cables out to a considerable number of points about the city, some one of which would be quite near to each subscriber. From each of these points to the various subscribers run short stretches of ordinary house-top wire. In this way hundreds of single wires would be gathered into small and inoffensive cables, and the enormous wooden structures would be replaced by small cable supports of brick or iron. In no place would there be the offensive multiplicity of wires. Such a system would be more durable, needing fewer repairs, than the present, and would not be much more expensive. For any other apparatus than telephones, retardation and induction would not be felt on so short cables. With telephone cables of moderate length these troubles would not be serious, and, if longer cables were necessary, metallic circuits could be used.