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Insulation

Excerpted from History, Theory, and Practice of the Electric Telegraph, 4th ed.
by George B. Prescott, Boston, 1866, as reprinted in 1972 by Frank Jones.

A discussion of U-981 "Elliott Hat" Insulator

Courtesy Steve McCollum

One of the most important matters connected with the electric telegraph is the proper insulation of the wires; but this, we are sorry to say, is one of the great defects of all the lines in this country. It does not matter how perfect our apparatus is in other respects; if the insulation is defective, it is a constant source of annoyance, and causes, oftentimes, great loss of business. Much can be done by increasing the power of the batteries, and by distributing them along the line; still the disagreeable fact ought not to be withheld, that in rainy or foggy weather not one of our telegraph lines in this country is reliable, or, if they work at all, it is only from one short station to another, and that with much difficulty. But this is also the case in England, France, Germany, - in a word, in every country where the electric telegraph has been introduced.

Cannot the insulation be improved, and something approaching the desideratum long hoped for be obtained? We think it can. Science and experience have been teaching us, ever since the first rod of telegraph wire has been in operation, that we should not rely upon glass as an insulator; and yet it has been almost universally used in this country. Every one has observed that, whenever the weather is wet or foggy, every article of glass is covered with a thin film of water; and of course each insulator on a line of telegraph is so covered with moisture. Certainly some electricity will escape over each glass insulator so covered; in fact, glass becomes a conductor as soon as it is exposed to humidity; it attracts to its surface the aqueous vapors of the atmosphere; they form there a thin film of water, by which the electricity passes away. When we reflect, that, on a line of telegraph 500 miles in length, there are 15,000 such imperfect insulators to conduct the fluid from the wire, we are at no loss to account for the dissipation of all, or nearly all, the galvanism generated by the battery, and the consequent bad working of the line.

The insulators now in use are the glass, unprotected by iron or other covering; glass protected by an iron covering; pine wood baked and soaked with shellac, and having a piece of glass inserted; glazed porous earthenware, or baked clay; glass upon wooden pins, protected by a wooden shield; white flint; bone-rubber surrounding an iron hook, the rubber having a screw cut upon it which is fastened to a wooden block; and the bone-rubber protected by an iron covering.

The chief and insurmountable objection to the use of the unprotected glass insulator is its great liability to fracture. So great does this objection practically prove, especially in thinly populated districts, where blows from missiles are most liable to occur, that on lines of one hundred miles in length, insulated with unprotected glass, there are always from five to thirty insulators fractured and useless. Within the past year we have been obliged to replace over one thousand of these unprotected glass insulators upon a line of one hundred and fifty miles in length. It is obvious that during rain-storms the working of a line thus imperfectly insulated must be sadly interrupted. The manufacturers of the glass insulator find it extremely difficult greatly to increase the strength of the material by increasing its thickness, on account of the difficulty experienced in suitably annealing it. A slight scratch will often cause the thicker insulator to fracture and become useless.

The iron-protected insulator (that is, a glass insulator with an iron covering) is practically much superior to unprotected glass, and is the kind at present used upon some lines. Still, it is in several respects highly objectionable. The glass within the iron is extremely liable to fracture from the effects of missiles striking the iron protection. When the glass within is fractured or cracked, capillary attraction ensues during moist states of the atmosphere, and thus is formed an electrical connection between the two metallic surfaces which the glass should insulate. When one of the iron-protected insulators becomes imperfect, it is extremely difficult, in riding by, to find the exact location of the difficulty, and determine which insulator is at fault. The large majority of posts splintered by the effects of lightning during the spring and summer months are upon lines making use of iron-protected insulators. The cause is obvious. The resistance of so large a mass of metal serves to attract and accumulate a great quantity of free electricity, which, having no conductor to the earth, except the damp post, and that offering a great resistance to its passage, is shivered in the descent. The iron protected insulator is necessarily costly, being composed of two materials, and manufactured at places usually quite distant. Double handling, and an extra transportation, augment the expense.

The chief objections to wood, coated or saturated with shellac, are, that the shellac cracks and decomposes upon the surface on exposure to atmospheric influences and during moist weather, and the difficulty of shaping wood into forms most approved for shedding rain, without large expense.

Porous earthenware and baked-clay insulators are principally defective from the fact, that the body is so porous as readily and easily to absorb moisture. Whenever the glazing is broken through by the wire and the spike, a moist communication is at once established, and the insulator is highly imperfect. A similar objection holds against the use of gums, resins, and other non-conducting substances less hard than glass, as the wire would soon wear through and touch the pin upon which the insulator rests; the surface, also, is liable to gradual decomposition on exposure. This system of insulation is extensively used in Great Britain, where they have undergone great expense in providing wooden roofs to shelter them from the rain; but they answer a very indifferent purpose, even when so protected.

In Germany and France they use glass very extensively, as indeed they do in Australia, California, South America, Canada, and other British American Provinces, and in the United States.

The glass with a wooden shield (Fig. 78) is preferable to the iron, and far preferable to the unprotected glass insulator; but it is open to the serious objection, that when a fracture occurs it cannot be discovered except by climbing the pole and making a minute examination (Fig. 79). We have under our charge some two hundred miles of line insulated in this manner, which has been in operation about four years, and has required very little repair; but it has not proved a good insulator in wet weather, it being difficult sometimes, after a few hours' rain, to work through a circuit of less than a hundred miles.

The white flint insulator (Fig. 80), invented by Mr. E. B. Elliott, we consider the best in use. The superiority of this insulator consists in its strength, insulating properties, and economy. The claim for invention is twofold: the anti-porous nature of the material - being the result of a continued series of careful experiments, and the improvements in form - tending to give increased strength, and also affording additional protection against the ill effects of atmospheric changes.

Its composition is flint, feldspar, and quartz, principally flint. The strength of the heaviest of the insulators in use has been thoroughly tested by pistol-balls, and also by being brought forcibly in contact with the iron-protected insulators in common use; in the former case merely flattening the bullets upon the convex surface, without fracturing the insulator, and in the latter case breaking not only the glass within the iron, but fracturing the iron protection itself. Its insulating properties equal those of the purest glass, and are superior to those of glass in common use. No deliquescent salts enter into its composition; consequently moisture from the atmosphere is less likely to accumulate upon the surface of this insulator than upon those made of common glass. Its anti-porous qualities have been made the subject of severest tests by scientific and practical telegraphers. The corrugations beneath serve as additional protection against such an accumulation of moisture as would permit a premature escape of the electric current, since the moisture is little likely to collect, at one and the same time, on the ridges and in the depressions. Hence, there is left upon the surface at least one dry and insulating ring, which the electricity cannot pass. As regards economy, this insulator can be afforded at a lower price than any protected insulator heretofore used, - the prices ranging from ten to fifteen cents each, being determined by the quantity used and by the pattern adopted. Adding to these prices six cents, the cost of an iron spike upon which the insulator is placed, gives from sixteen to twenty-one cents for the total cost of insulator and spike. If wooden pins, costing one cent each, are used, the total cost of insulator and pin will be from eleven to sixteen cents. The cost of iron-protected insulators now in use varies from twenty-five to sixty-two cents. There are, however, but few lines still retaining the iron-protected insulator, and they are mainly confined to use in cities, where the wires cross high buildings.

The white-flint insulator is fractured with very great difficulty, and would be uninjured by missiles as ordinarily thrown; but should one become imperfect, the fact would be at once evident, and could be detected as far as the insulator could be distinguished. It serves rather to protect posts from the injurious effects of lightning (which iron-capped insulators invite), and on this account is peculiarly adapted to the wants of the South and our Western prairies. Its insulating properties depend on no mere glazed coating. Its shape is perfectly adapted to the attainment of the greatest strength consistent with the affording of due protection from the injurious effects of storms and atmospheric influences. Should fracture occur, it would, in the majority of cases, continue supporting the wire, and be still an insulator. It affords the readiest facilities for prompt repair, an advantage of no small practical importance. It is throughout an insulator, and, like glass, impervious to moisture, under common atmospheric pressures.

This insulator has been used to some extent during the past five years, and experience has demonstrated the correctness of the statements we have made in regard to it; and although it is not a perfect insulator, it approaches nearer the requirements of such than any other which has been tried. Upon some lines, of forty to sixty miles in length, the operators have not experienced any escape during the heaviest storms. But to give them a thorough trial, they ought to be placed upon lines of several hundred miles in length. It is greatly to be hoped, that, in the changes now going on so extensively among the leading telegraph interests in this country, this most important and vital question of insulation may be placed in the hands of some thorough, practical electrician for solution. Our present system of insulation is a positive disgrace to the scientific ability of our American telegraphic engineers. Our principal lines work very well during dry weather, when in fact scarcely any insulation, beyond the dry poles, is needed; but let a shower, even, come up, and all the wires are seriously affected by escape.

It is not an infrequent occurrence, during the rainy season, for all communication between the important cities of New York and Boston, by the wires, to be suspended, notwithstanding there are no less than eight direct lines extending between the two places. There is no necessity for this whatever; and were the large and most approved pattern of white-flint insulator - set upon a wooden pin - substituted for the defective varieties now in use, we feel assured that the serious difficulty would be surmounted, and that telegraphic intercourse between these points would be placed beyond the reach of rain-storms or fogs.

In using the white-flint insulator, we should strongly recommend the use of wooden brackets for supports, instead of iron.

During the past few years a new material has demanded the attention of our telegraph managers, as a substitute for glass and other insulating substances, viz. bone-rubber. Some ten thousand miles of wire are insulated with this substance at the present time.

The form most generally used is that of a straight shank, terminated with a hook. The shank is of iron, covered with bone-rubber, upon which is cut a screw-thread, which is then screwed into a wooden block (Fig. 81), four inches square; the block is made fast to the posts by means of iron spikes. The wire is held by the hook, which depends from the under side of the block.

This insulator has but one substantial fault, and that easily remedied,-want of insulating distance. There is but one inch of insulating distance between the iron hook and the wooden block, and, of course, during a heavy rain-storm this soon becomes covered with moisture, and the current escapes in large quantities. In fact, with a good earth wire in our hands, and applying the tongue to the moist pole, we can taste the escape current within a few inches of the block.

A very large number of these insulators have been used upon the Northern and Eastern lines, and various expedients proposed to remedy the defect. One of them is a rather complicated arrangement, by which a glass cylinder is fastened to the hook, and the line wire tied to the glass. Besides being very easily broken, the glass is so filled and tied about with iron, as to afford scarcely any insulating surface.

We have proposed to remedy the defect of this insulator by simply increasing the insulating surface of the bone-rubber. Instead of having only one inch of insulating distance, let us have twelve. This can be done either by increasing the length of the shank, or by having the shank covered with a shield of bone-rubber. This would make a firm, durable insulator, giving twelve inches of insulating distance, six of which would be beyond the influence of descending moisture.

We cannot speak from positive knowledge, - to be only obtained, as are all scientific facts, by actual experiment, - but we have no doubt that an insulator thus made would enable a wire to work five hundred miles during any weather.

This insulator would be somewhat more costly than the ordinary kind, but not sufficiently so to bar its use from any line where good insulation is required.

A line of fifty miles in length may be worked in ordinary weather without the aid of other insulators than the posts, and it may be worked half that distance even during a storm; but this distance cannot be safely increased. In 1847, a line was constructed between Boston and Portland, -a distance of one hundred and ten miles, - without insulators, the wire being simply attached by iron spikes to the posts; but the experiment did not prove a successful one, the line not being able to work until insulated. The very idea of trying to work a line this distance without insulation showed most unpardonable ignorance on the part of the proprietor, the necessity for such insulation having been demonstrated by Franklin, Watson, and Le Mounier, for high tension electricity, in 1747 - 50, and by Gauss and Weber, Steinheil, and Wheatstone, in 1833-37, for that of low tension, or galvanism.

The use of iron, or other metals, in the construction of telegraphs, except as conductors, should be avoided as much as possible. Many lines have been seriously injured by the improper use of rods of iron, extending between posts situated upon opposite sides of a street, to which were attached insulators, for the purpose of conducting wires out of the branches of trees.

Such rods have been used in many of the cities and villages in Connecticut, where there are great numbers of shade-trees; but it was found that in damp weather they caused great escape from one wire to another, and they were consequently removed.

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Last updated Thursday, June 10, 2004