The Resistance of Insulators

[From The Telegraphic Journal.]

The Resistance of Insulators.

THE resistance of the dielectric or insulator presents some of the most important problems of electro-telegraphy, and it will be impossible that we can exhaust their application until we have attained to some acquaintance with instrumental work; but there are general principles, the knowledge of which will tend to the easier understanding of what we shall presently have to say. The specific resistance of insulators varies with every variation in their composition; and, as their composition is necessarily subject to many more variations than are likely to occur with a simple metal, such as copper, the tabulation of a series of resistances would be of little value to the student. Variation in the resistance will occur, too, through difference of age of the insulator, especially if this be India rubber or gutta percha, and difference of temperature results in difference of resistance of insulators to a very large amount.

But were these causes of variation eliminated, there would remain a very probable source of error in the difficulty of ascertaining how much electricity has passed by the surface moisture or dirt adhering to the insulator. This conduction by the surface of a body it is very difficult to avoid, and for the following reason: There is an exceedingly large quantity of common salt (or chloride of sodium) floating about in fine particles in the air. The perspiration of one's band contains a large proportion of this ingredient. Now, a great deal of this salt is deposited or condensed on the surface of any solid body, and as salt is, in relation to an insulator, not such a very bad conductor, a large proportion of the electricity escapes over the surface. Glass, when thoroughly warm and dry, is a very perfect insulator; but in ordinary or moist atmosphere much of the electricity finds its way to earth and is lost. The ebonite or vulcanite insulators of electrical instruments are very quickly covered with such a conducting film, and Mr. Parley, who has studied the subject extensively, recommends the washing of the surface of the ebonite or vulcanite with very dilute sulphuric acid, then with water, rubbing until dry, when the surface may have solid paraffin rubbed over it.

The presence of this conducting film at once explains how preferable it is to reduce the extent of surface of an insulator, on the ground that by reducing the superficial area we reduce the liability of the electricity to escape. The glass insulators of our telegraph posts are constructed on the principle of exposing as little conducting surface as possible.

It should be remembered that, although there is much difference between what is generally termed a bad conductor and an insulator, the latter does not mean a substance giving infinite resistance, through which no electricity can pass, but simply a very bad conductor. Graphite and gas coke are bad conductors, for their specific resistance is from 1,500 to 90,000 times that of pure copper. The resistance of gutta percha, however, bears very little relation to this, as we may gather from the resistances of a cubic metre of various kinds of gutta percha employed in the undermentioned cables:

B. A. UNITS OF RESISTANCE.

Red Sea ......................................................................................0.362+10 1/2

Malta-Alexandria.....................................................................................1.230+10 1/2

Persian Gulf.......................................................................................1.800+10 1/2

Atlantic (2d).......................................................................................3.420+10 1/2

Gutta percha at 24 deg C................................................................................3.530+10 1/2

For the purpose of comparison it may be stated that the British Association unit of resistance would be measured by a conductor of pure mercury — 1 square millim. in section, and 1.0468 meters long at 0 deg C. Of course, a cubic metre of copper would have no resistance.