Publication: The Telegraphic Journal and Electrical Review
SOCIETY OF TELEGRAPH ENGINEERS.
In France, a few years ago, an insulator of a very inferior character was employed. Since then, however, the French have assimilated their systems to those of other European countries, having adopted a form of double-cup insulator for all important circuits, and one with a single cup for less important lines, viz., for lines varying from 50 to 200 kilometers in length.
In Spain, Siemens' insulators were, until recently, almost wholly employed, but now the Prussian pattern, slightly modified, so that a groove in the top of the insulator supports the wire, is used. The groove is necessary, because in that country the old system of winders is still in existence. Insulators of the Prussian form, fitted on the upper extremities with two winding drums, are placed at intervals of one kilometre apart, the line wire running loosely through the grooves in the intermediate insulators.
In America, glass, as has already been mentioned, is almost wholly employed. The form mostly adopted consists of a single glass cup, in shape somewhat similar to the outer cup of a Varley's insulator. The interior of the cup is fitted with a female screw, into which is fixed a wooden pin, which supports the insulator.
An insulator called the Kenosha, consisting of white wood saturated with an insulating compound, the top protected with a metal cup, has likewise been somewhat largely used.
Brook's insulator consists of a cylindrical iron case, in which is inserted a blown glass bottle of peculiar form. Inside this is fastened a pin, which, terminating in a species of double hook, forms a support for the wire. It is claimed that a surface of blown glass, which is cooled by air contact alone, offers particular advantages in resisting deposits of moisture and dirt. This insulator has given some very good results, but it has not been practically tried in Europe against the forms in general use.
Testing. — The following is the method of testing at the chief factory of the Post Office in London.
All insulators are deposited in tanks, and filled with water, both inside and out, to within three-quarters of an inch to the lips. To prevent surface leakage, which would always exist in damp weather, they are kept dry on the edges by the heat of numerous jets of lighted gas, placed just above them, or the same result may be obtained by keeping the atmosphere of the testing-room artificially dry by means of hot-water pipes. After soaking for twelve hours, they are tested by means of a Thomson's galvanometer and 140 Darnell's cells. An ingenious combination of three keys and shunts is employed. In the normal position the galvanometer is short circuited. On depressing No. 1 key, 9999/10000 of the current is shunted from the instrument. No. 2 key introduces a shunt of 99/100, and No. 3 gives the full current. If a deflection be given with No. 1 key, the insulator is at once rejected, unless the lips are observed to be wet. If the latter be the case, or if deflections be given on depressing keys No. 2 or 3, the insulator is marked, and subsequently dried and carefully re-tested. Should any leakage still be shown, it is then finally rejected. By this means all defects of manufacture or accidental flaws, are inevitably detected before the materials are passed into actual employment.
In France a somewhat similar system is adopted, the standard being 7,000 megohms.
A series of experiments had been carried out by Mr. Gavey in which the insulation of various sections of line had been compared with the hygrometrical condition of the atmosphere, and from which it appeared.
First, that there is as it were a great wave of moisture that sweeps daily over the land, having its maximum near midnight, and its minimum at noon; and that accordingly the insulation of our circuits, generally low from 7 to 9 a.m., rises to a maximum from 11 a.m. to 3 p.m., then abruptly falls, reaching a minimum between 7 p.m. and midnight.
Secondly, that very frequently the most abrupt changes in insulation take place in a most limited time; the resistance dropping from several megohms to a fraction of a megohm per mile in the course of an hour or two. Further experiments had been made in which the various forms of insulators were compared among themselves. The subjoined table gives the result of these trials.
Thirdly, that the resemblance between the insulation and moisture curves is very remarkable; so much so, in fact, that it is not improbable that an extended series of insulation readings would form a more accurate register of moisture over any given extent of country than ordinary hygrometrical readings. Further experiments had been made in which the various forms of insulators were compared among themselves. The subjoined table gives the result of these trials.
When considering the advantages of any description of insulator, it is evident that one may have a better form for insulation under all circumstances than another of an inferior shape, but it may give an equal or lesser absolute resistance, owing to the latter having a greater length, or lesser section than the former. It therefore becomes desirable to reduce the absolute results, obtained in a series of tests for such as those recorded, to an unit result, so that a clear conception of the value of form may be obtained, and, if necessary, increased length or diminished section be applied to get the best results. This is done in the table in the following manner. The lengths of each cup are divided by the mean circumference, both inner and outer, and the absolute resistances are multiplied by the fraction —length by circumference—to give the "reduced" or "form" resistance of each insulator.