Publication: The Telegraphic Journal
ON TELEGRAPH CONSTRUCTION.
By JOHN GAVEY.
(Continued from page 238.)
THE two principal points for consideration in tho selection of an insulator, are its form, and the material of which it is composed.
These have been varied in a multitude of ways, but certain definite principles can be laid down, which are of more or less universal application.
First, the electrical qualities require attention, the object being to obtain an insulator which opposes the greatest possible resistance to the passage of a current of electricity. Conductivity is possible either through the mass of an insulator or through a film of moisture or conducting material over its surface. To provide against the former, a material which as nearly as possible odors an infinite resistance should be selected. It should be homogenous, uniform in quality, and not porous, or in any way un absorbent of moisture. It should not deteriorate crack, or split, by exposure to atmospheric influences, but should retain permanently the qualities sought for in its selection.
Surface conduction arises from the deposition of a film of moisture, or of the more or less conducting substances which float about in the atmosphere. The resistance of an insulator to loss from surface conduction depends primarily upon the form given to it, and in deciding this point, the ordinary law that the resistance of a conductor varies directly as the length, and inversely as the section, affords a guide as to the dimensions that should be adopted; any means by which the length of the insulator is increased and the diameter diminished increasing its resistance per se— the affinity for moisture, however, of the material employed, the smoothness and evenness of the surface, the readiness with which dirt adheres to it, and insects form their nests within the interior, and the facility with which the surface can be washed by rain, all tend to vary the results obtained with different forms and materials, and should all be carefully taken into account in considering the relative value of various forms employed by different services.
The materials used are varied, and the qualities to be sought for, in addition to those already specified, are sufficient tensile and compressive strength to withstand with success the greatest strain likely to be brought on them. They should be readily moulded or wrought into the desired shape, and should be tough and adapted to resist a certain amount of rough usage to which they will Inevitably be subjected. Glass, stoneware, porcelain, ebonite, and wood impregnated with insulating compounds have all been used with various degrees of success. Glass was widely used in England in the early days of telegraphy, but the readiness with which moisture was deposited on its surface, and its liability to crack, caused it to be rapidly replaced by stoneware and porcelain, which are the two materials wholly used in this country It is possible, however, that its use may be revived with great advantage if the electrical qualities o the toughened glass, of which so much has lately been heard, are at all comparable with its allege' mechanical ones.
Stoneware and porcelain, the former especially are excellent materials for insulators. If selected with care, properly manipulated, thoroughly burn and vitrified throughout their whole substance they are proof against nil ordinary vicissitudes except that of mechanical violence, to which unfortunately they are most subject. The materials are readily moulded into any desired form, and a clean, well glazed, thoroughly even surface is easily obtained. Ebonite, one of the most perfect of insulators as a mass, soon deteriorates over it surface, which becomes partly conducting through the adherence of dirt. Wood has been used a giving a substance which can withstand with impunity the stone-throwing proclivities of mischievous persons. The difficulty with this material, however, is to give it that thoroughly smooth surface which is of such importance to the continued efficiency of a line.
The earliest practical insulator that was used to any extent in this country simply consisted of a short cylinder of glazed earthenware, with a hole through its axis and a groove round its circumference. It was held to the pole by means of a small clip, fitting the groove, and the wire passed through the axial hole in the earthenware.
This was shortly afterwards modified, the cylinder being lengthened and the ends tapered in he form of a double cone. Later, with the object of diminishing the deposition of moisture on the earthenware the latter was formed into a solid cylinder, perforated at one extremity at right angles to its axis for the passage of the wire, and covered at the upper cud by a 2inc cap, which surrounded the greater portion of it. Although this achieved its object, the remedy was as bad as the disease, for the cap, without in itself impeding the escape of the current, fostered the accumulation of dirt and spiders' nests, so that the insulation left much to be desired.
This was followed by the umbrella-shaped insulator, known to old telegraphists as the ''No. 3," which was a direct descendant of the last form, the zinc cap being replaced by an earthenware or glass one. This insulator did good service, but as the system extended, and longer lines became common, an improvement was found necessary. This was partly achieved by paying greater attention to the manufacture of the material, but the greatest advance was made in the form given to the insulator, which is a type of those now in use. This was introduced by Mr. Latimer Clark, who in 1856 patented his double cup insulator, which consisted of a double boll cemented on a bolt or stalk, by which it was attached to a bracket, the wire itself being supported by a suitable groove on the top of the insulator. Mr. Varley later improved on this by introducing an insulator in which a long narrow cylindrical cup was cemented inside of a larger one, so as to have a clear space between the two. A suitable bolt or stalk, to form an attachment to the pole, was fastened in the small cup, and the large one contained a circular groove near its upper end to retain the wire. By these moans the mean section of the conducting surface was diminished, whilst the length was considerably increased. Further, by the use of two separate and independent cups, the additional advantage was secured, that a breakage or undetected defect in one of them simply reduced the effectiveness of the insulator, by say one half, instead of almost wholly neutralizing its insulating properties. For short lines single cup insulators, similar in outward form to the above two descriptions, are now used, but the former or modifications of the same principle are now employed on all important circuits in most countries. Occasionally the insulators are protected against mechanical injury, by being covered with an outer iron cap. This, however, bus a most injurious effect on the insulation. as it fosters the accumulation of conducting matter over the surface of the porcelain or earthenware, and impedes the beneficial action of rain, which in the ordinary type cleanses the exposed surfaces whenever it falls heavily. To obviate this, perforated cups have been introduced, which, whilst protecting the brittle material from stones of a size to injure them, admit of the entrance of sufficient water during heavy rains to wash the insulators. Even these are, however, far inferior in insulating power to the ordinary unprotected form.
In testing insulators, they should be exposed to precisely the same influences as they will be subject to in actual practice, or fallacious results will be obtained. The minimum insulation resistance of an open line of telegraph in the worst weather, is generally taken at 200,000 ohms per mile. Thus, taking 30 insulators per mile as the maximum number which will be used at any time, a single one should not offer a less resistance than say 6 megohms after exposure for some months, and in wet and misty weather. Practically a higher result than this even should be given to counterbalance the effect of exceptional losses in the weak places which every long line exposes. Some years ago Mr. Culley, for the Electric and International Telegraph Company, inaugurated an exhaustive series of experiments on the value of various classes of insulators, which resulted in the adoption of the forms now in use in this country. Those who seek further details on the whole subject, are referred to his handbook, where it is treated of at length.