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 samples of gutta-percha and India-rubber coated wires, I found a marked difference in each variety; but as these differences were of a constant character for each sample, it was easy to tell which of these had the highest insulating property.

These experiments showed me that if I wished to arrive at the true value of an insulating oil, I must make the experiments by a method by which the result should be entirely free from any interior or exterior coating. For this I took two small flat circular discs, one inch in diameter, which could be immersed in oil, and by an insulated adjustable screw I could compare the striking distance of these discs in air, compared with the same when immersed in oil, and could also observe the time required for a complete discharge of the Leyden jars through this medium.

On testing numerous samples of different oils, I found not only a great difference in the species of oil, but also a great difference in different samples of the same oil; consequently a table giving the results on different oils might be misleading, as a sample of superior quality of a certain oil (although inferior in many other samples) might give higher results than a badly selected sample of a really superior oil.

The only difficulty as regards rosin oil is one of manufacture, or, rather, of finding, and teaching the makers, the quality of oil best suited for the purpose; for I found, on obtaining samples of this oil from different makers, that a great difference existed as regards their insulating properties, ranging from worse than castor oil up to a degree superior to gutta-percha; and this is true of most oils; consequently, before using any oil, its quality as an insulator should be thoroughly known by constant electrical tests.

In selecting oil of high insulating quality, we must also have regard to the purpose for which it is to be used. Thus, as a self-restoring medium having very quick action, for condensers, transformers, or coils so closely wound as to be difficult for a thick oil to penetrate, a thin rosin oil, such as rosin spirit, might be best ; but for cables and underground wires I found thick pure rosin oil best, because it was not only superior as an insulator, but it would not escape too rapidly at any large puncture.

Experiments were also carried on at the same time to observe the effect of any given oil on thin sheets of gutta-percha, India-rubber, &c. These were immersed in separate vases of different oils: they were weighed before and after prolonged immersion; the result being that some oils were found to be injurious to gutta-percha, and almost all, with the exception of castor oil, were more or less destructive to India-rubber.

I found, after numerous experiments, that pure rosin oil gave the highest insulation of all, for a spark that would pierce a given thickness of gutta-percha would utterly fail to pierce the same thickness of rosin oil; whilst, if the gutta-percha was pierced, its insulation was destroyed and could not be easily restored to its former condition; on the other hand, if by accident the rosin oil was pierced, it, by its own mobility, at once restored the insulation to its original state. Rosin oil, at the same time, had a preservative effect upon gutta-percha, for the sheets immersed in this oil had become slightly increased in weight, showing that it had penetrated into the pores of the gutta-percha; at the same time it was stiffer and tougher than before immersion. I will now show you an actual specimen of this fluid cable, cut off from a specimen one mile in length, made by the Gutta-Percha Company 33 years since. You will observe that the gutta-percha has absorbed during that time a large proportion of the oil, and that the gutta-percha is now, after 33 years' exposure to the air, apparently as good an insulator as when first made.

Experiments were now made as to the value of rosin oil when employed as the sole insulator of an electric cable. For this purpose, a short length of copper wire (say 1 foot) was coated, first, by being wound by well-dried cotton or hemp, or, better still, by a string wound round in wide open spirals, in order that the oil should penetrate as freely everywhere as possible; this was afterwards drawn into a small lead tube filled with rosin oil : thus the string or fibrous covering on the wire was simply to keep the wire concentric, so as to prevent its touching any part of the exterior tube without some separation of oil or oil-saturated fibre. It will be seen that these tests were identical, both as to form and material, in their essential points, with those patented by Mr. David Brooks, of Philadelphia, some fifteen years later; but as these are clearly described in my patent of January 11th, 1859, there can be no question of priority in this respect, though no doubt many improvements suggested by practical experience in the mode of laying and manufacture are original with Mr. Brooks.

The insulation of this form of cable was not quite as high as solid gutta-percha, due to the fibre not being as good an insulator as the enclosed oil, but its mechanical powers of self-restoration and durability seemed to me to more than counterbalance its slightly less insulating power, for even with gutta-percha there are questions of strength and durability which ought never to be lost sight of in favour of a temporary high insulation; and I think that when the value of fluid insulation is more thoroughly known, especially for currents of high potential, it will be more generally used than at present, particularly as there are now no valid patent rights to interfere.

I again repeated these experiments, using a thinly covered gutta-percha wire for the inner conductor, drawn into a gutta-percha tube full of rosin oil; this gave me very high insulation—so much so, that I tried a bare copper wire in the gutta-percha tube alone, without any rosin oil; this, to my surprise, gave a far higher insulation than the best gutta-percha-covered wires as supplied to me by the Gutta-Percha Company. This surprised me very much, as the tubes, which I bought at an ordinary retail shop, were known to be of an inferior quality of gutta-percha. I found afterwards that this difference was due to the tubes having been long made, and kept in stock: they had gradually dried, and were free from combined moisture absorbed during their manufacture. I proved this by taking some newly manufactured gutta-percha, and heating it gradually for a length of time sufficient to drive off its moisture: this when modeled on a wire, so as to form a cable, gave precisely the same degree of high insulation as the old tubes that I had purchased elsewhere.

I do not believe that the mechanical quality of the gutta-percha was improved by this operation; most likely it would have soon become brittle, for humidity, or an essential oil, seems necessary to its life and mechanical qualities.

I told Mr. Chartterton and Mr. Willoughby Smith, of the Gutta-Percha Company, of these results, and urged upon them the necessity of well drying the gutta-percha, and of manufacturing their cables, if possible, without constantly macerating it in water. I am not aware that my remarks had any effect, but this I remember—that in a few weeks after, they gave me a sample, which they called "special gutta-percha," which certainly had as good insulating qualities as my dried gutta-percha.

As I have already said, my main object being to produce a self-restoring insulation for cables and underground wires, I found that a thin fluid, such as rosin spirit or petroleum, might be objectionable, as at the point of fracture or puncture the fluid might escape with so great a rapidity as to be somewhat costly in its maintenance ; but by employing a thick, heavy, insulating oil, whose rate of flow would be small, this would displace any water in the puncture, provided that there was a slight head pressure given to the oil at a reservoir at certain landing stations: then it would answer all practicable purposes. For this I preferred rosin oil, which is already a thick, viscid oil, and can be made more so by the addition of solid rosin dissolved in it, or by the addition of palm oil residue, which has a remarkable property of thickening rosin oil.

In order to satisfy those who might object to even a very small leakage of oil at the puncture, I made several compounds of rosin oil, mentioned in my patent, that should harden when in contact with water, and thus prevent any waste of oil at the fault or puncture.

The static charge which I used was of high potential; the sparking distance in air, of the charge used, was about 1 inch, and it showed the remarkable insulation of rosin oil when 1/64 th of an inch separation would effectually prevent such a spark passing through it. After having found this high insulating property of rosin oil, I coated all my Leyden jars, and all parts where I desired high insulation with it, and by this means I was enabled to retain a full charge of the Leyden jars for several hours in an atmosphere full of humidity.

They have lately proved the value of oil insulation in Frankfort by the use of oil in transformers of 20,000 volts, and I am convinced that in all cases where we need a high insulation, together with the power of self-restoration, it can only be found in fluid insulators. It is useful in every case where it can be applied; it is far cheaper than gutta-percha, and I have no doubt that it will soon be used for insulation in condensers as well as transformers. Mr. Nikola Tesla, in his late charming lecture, spoke highly of the use of oil in his trans- formers, saying that it not only gave a perfect insulation, but had the power of restoring the insulation whenever pierced or punctured, as he believed his coils were at least several times every day.

I remarked during these experiments that, no matter how high the potential, the amount of leakage was equal in time, either with a high or low charge of the Leyden jars; that is to say, when the Leyden jars were discharging at a slow rate through a good insulator, the time of falling of the electrometer through any given number of degrees was very equal throughout its whole range from 200,000 to 1,000 volts.

Being desirous of repeating this evening some of my old experiments, I applied to Mr. W. H. Preece for some of the best samples of gutta-percha covered wires as used by the Post Office, and which he has most willingly sent me. I also applied to Mr. James Wimshurst, the inventor of his remarkable static electric machine, who at once most kindly consented to lend the machine, and also design an arrangement by means of which we could show to those at a distance the different distances of the sparking through oil compared with air.

On testing this apparatus with oil, it seemed to show that the value of oil as an insulator increased both with the potential of the charge and also with the rapidity of alternations, for when the experiments were made by an impulsive rush so as to have the greatest number of oscillations per second—say 1,000,000—then even the poorest oil, such as castor oil, showed marked superiority over air or gutta-percha. This confirms the advantage of oil, and its value in connection with the rapid alternating high voltage currents used in our transformers of to-day.

The experiments which we hope to repeat this evening have already shown us that a spark, or charge, from a Leyden jar, which would easily pierce 4 inches of air, will not pierce 1/19 th-inch of rosin oil, proving that oil, when resisting an impulsive rush of a charge, has 79 times higher insulation than air.

To obtain these results, we used a large and most powerful Wimshurst machine; but this evening, for convenience sake, we shall use a smaller machine. We may not be able to obtain quite as high comparative values, still we hope to obtain results showing oil to be 50 times better an insulator than air.

With the largest machine we obtained the following remarkable effects. The apparatus for holding the oil was a glass vase, 4 inches in diameter by 3 inches deep. The lower portion of this vase had a copper plate connected with one portion of the circuit. A brass knob, 1/2-inch diameter, connected with the other portion of the circuit, could be raised or lowered in this vase in order to show the different striking distances when the vase contained air or oil. This gave the comparative results already mentioned; but, in addition, we noticed that when the knob was only three-quarters covered with oil, the spark rose from the upper portion to the upper portion of the vase, and then descended on the outside in order to reach the lower copper plate—a course of some 5 inches through the air, compared with 1/20th-inch through oil. If now the current was increased, the current, or spark, became a continuous sheet flow all over the entire surface of the vase, resembling in appearance a waterfall, or exactly imitating the well-known Gassiot cascade. On again increasing the charge, in an attempt to pierce the oil, the spark pierced the glass vase, making a small hole of about 1/32nd-inch diameter, at a place ˝ inch above the surface of the oil; thus the spark preferred to pierce the glass and travel several inches through air, rather than traverse a distance of 1/19th-inch through oil.

I will now show several forms of oil insulation as applicable for submarine or subterraneous wires, and the remarkable power of self-restoration when the insulation is temporarily destroyed by a puncture or cut across, so as to lay the wire bare for an instant.

It will be seen in the experiments following this paper, that I have arranged a battery so that one pole is connected through a sensitive galvanometer to a metal tank containing salt water, the return circuit being completed through the salt water and wire to be tested to the other pole.

If we take any sample of gutta-percha-covered wire, and place it in the water, the insulation seems perfect and all that could be desired; but if we make a small incision with a knife, so as to leave a minute portion of the copper exposed, then (as we already know), on placing this in the tank, the water at once percolates through the cut to the wire, and its insulation is completely and permanently destroyed.

If we try this same experiment with oil cable, such as a bare copper wire in a gutta-percha tube containing oil, or, better still as a proof, a thinly coated gutta-percha wire, and this plunged into a small lead tube containing thick rosin oil, the whole having the same diameter as the gutta-percha-covered wires, we find, on testing this lead-covered oil cable, that its insulation is as perfect as the gutta-percha wires.

If we now cut through the lead tube and the gutta-percha wire once or several times so as to lay the wire bare, the insulation for an instant is destroyed, but in less than a second the oil percolates through the wound or cut, and its insulation is instantly restored to its previous high value.

I believe that these experiments sufficiently show the merits of a self-restoring insulating medium, such as rosin oil, for all purposes where a permanent insulation of wires conveying an electric current is desirable.

In conclusion, I will mention