Publication: The Telegraphic Journal and Electrical Review
Proceedings of Societies.
SOCIETY OF TELEGRAPH ENGINEERS.
AT the ordinary general meetings held on the 13th and 27th ults., a paper by Mr. J. Gavey was read, and a discussion ensued, on the subject of "Insulators for arial telegraph lines."
In dealing with the principles which should guide the selection of an insulator; the author treated his subject under two heads, viz., the material of which the insulator is to be composed, and the form which should be given to it.
(1.) The material should offer an electrical resistance as nearly as possible infinite; it should be homogeneous and non-porous throughout; it should be capable of taking a high polish or smoothly glued surface; it not be subject to deterioration, either externally or internally, through atmospheric or other causes; it should be readily moulded into any desired form; it should have as slight an affinity for water as possible, and its tensile and compressive strength and toughness should be sufficient to enable it to withstand the maximum strains to which it is liable to be subjected, as also the ordinary rough usage.
While on the subject of porosity, Mr. Gavey mentioned that he had made a series of experiments with various sun les of oolite, lias, alabaster, and sandstone, the conclusion he had arrived at being that only at temperatures far above ordinary atmospheric temperatures, is it possible to expel absorbed moisture from a porous substance.
The principal materials hitherto used for insulators are glass, porcelain, earthenware, ebonite and wood saturated with insulating compounds.
Glass, originally one of the first materials used, was rejected in this country, on account of its readily condensing films of moisture over its surface, and also because of its brittleness and its tendency to fracture under variations of temperature. It is now, however, used in America and Switzerland to a considerable extent. It possesses, in a high degree, many of the requisite qualities of a material for an insulator. Its electrical resistance is almost infinite. It is non-porous, homogeneous, highly polished on its surface, readily moulded into any given shape, and easily manufactured of a given quality, but unfortunately the objections already mentioned were found so strong as to lead to its entire rejection in England.
Porcelain has perhaps been more widely adopted than any other material, and, if properly selected, well manufactured, thoroughly burnt, so that a partial vitrification takes place throughout, it becomes homogeneous and non-porous, probably affording the best material hitherto used. If of really good quality it satisfies many of the conditions laid down, its principal defect being its brittleness. Of course much depends on the selection of the ingredients of which it is made and on the care devoted to its manufacture.
According to the description of clay used, and the different proportions in which this is mixed with the powdered flint and other materials employed, will the porcelain be bard or soft. The proportions used are generally trade secrets, each maker having his special formula for mixing. That manufactured in Prussia, is of a very fine description, extremely hard, and is much liked.
In this country a softer porcelain generally is used for insulators. It is probable that the harder German porcelain gives a higher specific resistance than English porcelain generally, but the latter can be selected so to give such a high result as to be practically infinite. There is no evidence to show that the softer qualities deteriorate more rapidly under atmospheric or electrical influences than the harder ones.
The final test for any material used for insulators, is, of course, the electrical one: for invisible fissures, porosity, or other imperfections, are thereby detected; but a careful examination of the fractured sections of an insulator will generally give some idea of its quality. If properly made up and sufficiently fired, the fractured surfaces will be more or less conchoidal, smooth, and homogeneous in appearance. Any departure from this evidently indicates imperfection.
The following are the principal causes of the low resistance of porcelain insulators which are rejected after being exposed to the ordinary tests.
1st. Flints not ground fine enough to make with the other constituents of the porcelain a smooth paste. The fractured section of the ware, when examined with a lens, is roughly comparable to a quarry of chalk with its intermixed flints.
2nd. Insufficient firing to fuse the flux.
3rd. Excessive firing, which makes the ware spongy.
It will be seen that these are merely defects in manufacture, preventible by the adoption of proper precaution; and as they are readily detected they do not militate against the use of the material itself.
Stoneware, which has been employed to a considerable extent in this country, is perhaps of a more variable character than porcelain, but if carefully selected, moulded under a considerable pressure, and well burnt, it falls very little short of porcelain as a material for insulators.
Ebonite, which appeared a most promising material, and was at one period most extensively used, failed through its rapid surface deterioration under the influence of the atmosphere.
Wood, saturated with insulating substances, has been tried, but it is questionable whether a high-class insulator could with safety be turned out of this material. It is extremely difficult to deprive wood of its strong tendency for absorbing atmospheric moisture; in fact, it is a question whether it would be possible to entirely fill up its pores; and the freedom from malicious fracture, which its use would carry with it, would be dearly purchased at the cost of the lowered insulation which would certainly follow any failure in the attempt to render it impermeable.
(2.) The Form—The theoretical conditions to be aimed at in designing the form of an insulator may be enumerated as follows:--
The maximum resistance should be obtained by increasing the length to be traversed by the current, whilst diminishing the section of the conducting film; by the retention of a dry surface on one portion of the insulator, if possible, under all circumstances and by the adoption of a form that will not aid or retain deposits of dust, soot, or other materials which act injuriously by retaining and increasing the thickness of the moisture films.
Taking the resistance through the substance of the material selected as approximately infinite, which it would be practically in any but a defective specimen, the conductivity of an insulator arises through the deposition of a film of moisture over its surface. Now the ordinary law, that the resistance of a conductor varies directly as its length and inversely as its section will evidently apply in this case as in others, and therefore in calculating the resistance of a given insulator, we have to consider the distance over its surface from the point where the wire is attached to the point where the insulator is affixed to the pole; and also the thickness of the film of moisture multiplied by the mean circumference of the insulator. Assuming, therefore, that under like atmospheric conditions the thickness of the film of moisture deposited on various forms of insulators of the same material will be constant, the resistance of each insulator will vary directly as its length and inversely as its mean circumference. This law in practice applies with accuracy only to simple cylinders, for in complex forms of insulators other disturbing causes are introduced.
In the early days of telegraphy, various substances were tried (among which was a simple goose-quill) and improvements made, by which the standard of insulation was gradually raised, until in 1856 Mr. Latimer Clark introduced on the lines of the Electric and International Telegraph Company the well-known porcelain invert, which may be called the parent of a whole generation of the modern form at present in use. It consisted of a so-called double-cup insulator, supported on a vertical pin, and containing a deep in the top to carry the conductor. These were largely used in England at the time they were first made, but they were subsequently replaced by Varley's well-known earthenware insulator, in which the two cups were altered in shape, prepared in separate pieces, and cemented together, so that a flaw or fracture in one would not destroy the insulating power of the whole.
The postal telegraph department of Great Britain has lately introduced a new insulator of the double-cup form, in which the length is considerably increased, and which is attached to the bolt by means of a female screw in the interior of the cup, fitting a corresponding thread at the upper extremity of the bolt. An India-rubber washer, placed between the lower end of the insulator thread, and a flange on the bolt, prevents fracture of the insulator by over-screwing.
In Prussia the porcelain invert already referred to was adopted early after its invention, and the present form in use in that country may be said to be but a modification of the original, the weak parts being strengthened so as to better resist the effects of usage, and the sections lengthened to increase insulation. These insulators, or modifications of them, have been introduced in the German Confederation, Russia, Sweden, Denmark, Italy, and Spain. They are like- wise used by the Indian Government Telegraph Department.