Comparison of insulators

[Trade Journal]

Publication: Electrical World

New York, NY, United States
vol. 3, no. 11, p. 82, col. 2-3



The invention of hard rubber has perhaps done more to facilitate electrical progress than any other single discovery. This becomes apparent when we consider its extensive use in almost every kind of electrical apparatus.

Its value arises from its excellence as an insulator, the comparative ease with which it can be manipulated, and the elegant finish of which it is susceptible, as well as its strength and durability. On the lathe it can be reduced to almost any form of which wood or metal is susceptible, and receives a permanent polish, without varnish, laquer [sic] lacquer or plating. By heat and pressure it can be bent and molded into various forms. And it is not affected by moisture or acids.

On the other hand it has many defects. As an insulator it is inferior to many other substances. Its liability to soften and warp with heat and its comparative brittleness render special care necessary in manufacturing articles from it, and unfit it for many uses. It is often found to be porous and full of air holes, where it was apparently sound, and has to be rejected at a loss of time, labor and original cost. In use it is liable to become discolored after a time, and to deteriorate in quality, and when exposed to the weather, as for insulators on telegraph poles, it has to be rejected entirely.

In consideration, of these defects, and of the importance of an insulator which should be free from them and still retain the good qualities of hard rubber, it is surprising that so little progress has been made.

The production of gelatinized fibre and the claims made in its favor inspired the hope that the long-sought article had at last been found. But the first test with electricity of high tension annihilated its claims as an insulator, and left it doubtful on which side of the line between insulators and conductors it should be placed. The test was made with static electricity, and proved that the fibre as an insulator was inferior to kiln-dried wood, and far below hard rubber.

It may be found very useful for many purposes where good insulation is not essential, or where a conductor of high pressure is required. It is more tenacious and more easily worked on the lathe than the rubber, holds a screw-thread better, has a close, firm texture, and takes a high polish. As it closely resembles the rubber, is about the same weight, and only one-third the cost, it may become a dangerous counterfeit where insulation is required.

Compounds of asphaltum and marble dust, and shellac and marble dust have high insulating qualities, and do not easily collect moisture on the surface; but they are hard and brittle, like glass and porcelain, and hence unfit for lathe work.

In speaking of glass as an insulator, there is very seldom any discrimination as to quality, even by scientific writers of high repute; and there is a general impression that all glass insulates equally. The erroneousness of this impression will become apparent by attempting to make a Leyden jar of the ordinary "specie" jar used in candy shops, or of the "salt mouth" jar, used by druggists; both these jars are usually made of American flint glass, which is very clear, and this, added to their graceful form, makes them very desirable for this purpose, and just such as an inexperienced amateur would be likely to select. But any attempt to make Leyden jars of them will prove a failure, as the glass will not insulate static electricity. The same is true of battery jars made of American flint glass.

Without entering minutely into the chemical composition of the various kinds of glass it may be said that where lead or any metallic substance forms one or more of the ingredients the glass cannot be relied on for insulation, and this accounts for the imperfect insulation of American flint glass, composed, in part, of lead and sodium.

The best insulating glass is of a bright, sea green color. Such glass makes the best Leyden jars, and is doubtless nearly or quite