Oil as an Insulator

by Prof. D. E. Hughes, past-president of Institution of Electrical Engineers

[Trade Journal]

Publication: The Telegraphic Journal and Electrical Review

London, England
p. 321-323


Institution of Electrical Engineers.


Oil AS AN INSULATOR (illustrated by experiments). By Prof. D. E. HUGHES, F.R.S., past-president. (Announced for reading yesterday.)

At our last meeting, Mr. Preece kindly gave me credit for being the first to propose the use of oil as an insulator for wires conveying an electric current; and in accordance with a suggestion on his part, I should like to state upon what grounds I can fairly be considered to be the first to urge upon the electrical world the use of hydrocarbon oils, such as petroleum and rosin oil, for this purpose.

In 1858 universal regret was felt on the failure of the insulation of the first Atlantic cable. From the first successful laying of this cable the insulation gradually became worse, until in a few days all signals failed. The cause of this was supposed to be due to minute flaws in the gutta-percha during its manufacture, which became worse by submersion, or that lightning, or the intense currents then used for the Whitehouse induction coil, punctured the cable at several points.

It appeared to me (from some old experiments of mine) that what we needed was some form of insulation that possessed self-restoring powers, so that, if punctured by lightning, or our ordinary working currents, the puncture should be closed by some simpler process than having to take up a portion of the cable.

I thought that Nature showed us the way in which she restores punctures and mechanical injuries to all living objects. This is done by a flow of liquid sap to plants, or blood in animals; for if we make au incision in the bark of a tree, the sap flows out and hardens in contact with air; if we cut our fingers, blood flows out and heals the wound. Therefore, I thought that if I could enclose in a cable an insulating self-restoring medium, the cable would not become dead at the first, or even after innumerable punctures. To carry out this object it seemed to me that a thick insulating oil, enclosed between the wire and its outer skin, would perfectly fulfil the conditions required.

Faraday had shown some years previously that oil of turpentine had a high insulating property, and this gave me great hopes that I should find, by experimenting with numerous samples of oil, the kind and quality desired for my purpose. Therefore, I at once commenced a long series of experiments in order to find the best kind of oil and the most suitable form of cable to carry out this idea.

Knowing that I could not fairly test these oils, or a short length of cable, by our ordinary voltaic currents, I had recourse to the very high potential currents given by the ordinary frictional static electric machine. The method used was this: I charged a battery of Leyden jars to a known degree, which was indicated upon the Leyden jars by a Peltier electrometer; these jars, when charged, were put in communication with the short piece of the cable to be tested, the outside of which was coated with tinfoil, or placed in water connected with earth, and exterior of the Leyden jars. By this means, if the insulation of the few inches of cable was bad, the Leyden jars would instantly be discharged through the defective insulation; but if the insulation was comparatively perfect, the time of discharge of the electrometer was a correct measure of its insulating properties. Of course this method rquired that the Leyden jars should be perfectly insulated and hold their full charge for at least one hour when not discharging through a defective cable.

On making preliminary experiments on several