Popular Science Monthly/Volume 13/October 1878/Electricity in Thunder-Storms

by Elisha Foote

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ELECTRICITY IN THUNDER-STORMS.^[1]

By ELISHA FOOTE.

THE great development of electricity in thunder-storms has been a subject of much speculation. Its explanation, however, is still an unsettled question. Some views on this subject are presented in this paper.

We have no evidence that the production of fogs or clouds—the change from invisible to visible vapor, or from combined to uncombined moisture—produces any electricity. All experiments to establish such a supposition have had a negative result.

These particles of vapor we may suppose to be small spherules, each with its normal portion of electricity that surrounds or occupies the surface of the sphere. When two of these particles unite and form one, the combined particle will have twice the electricity of either of the separate parts, but not twice the surface. There will then be an accumulation of electricity upon the surface of the combined particle; and still more will this be so when thousands of these spherules unite to form a drop of water.

We may well conceive, therefore, that a cloud forming water should become surcharged with electricity, that will escape in violent explosions when the accumulation is too rapid or the circumstances are unfavorable to its being carried off by the surrounding moist air.

It is not, then, the formation of vapor, but its condensation to rain, that produces thunder and lightning. And this, it is believed, accords with all our experience. Clouds are constantly forming and disappearing; fogs and vapors are accumulated in some places in great abundance, but no electrical excitement has ever been observed. But, on the other hand, there is never a flash of lightning without a manifest deposition of rain. To this there is no exception. There is, indeed, a manifest relation between the two. The more sudden and rapid the condensation, the more violent and terrific the explosion.

Sometimes, in thunder-storms we hear a loud crash, and then, soon after, comes an increased pouring down of water. Sound travels more rapidly than rain, and, although the report reaches us first, the interval between the events and the distance traveled plainly indicate that the explosion succeeded the condensation; and we naturally infer that it was caused by it. The loud crash and simultaneous lightning show the nearness of the explosion, at the origin of the rain-drops.

I next inquire whether we have experimental proofs corroborating these views.

A few years since an accidental escape of steam from a steam-boiler was found to strongly electrify a person who stood upon an insulator and held one hand in the escaping steam. This excited much interest at the time, and it was investigated by Armstrong and others, and led to our present steam-boiler electrical machine. The phenomena were at first supposed to throw much light upon the causes of atmospheric electricity. The subject was subsequently taken up by Faraday, who instituted a series of experiments, and came to a different conclusion. His theory, using his own words, was:^[2]

"The electricity is due to the friction of the particles of water which the steam carries forward against the surrounding solid matter of the passage, . . . and is in its nature like any other ordinary case of excitement by friction."

Again (section 2145), he says:

"Finally, I may say that the cause of the evolution of electricity by the liberation of confined steam is not evaporation; and, further, being I believe friction, it bas no effect in producing and is not connected with the general electricity of the atmosphere."

The great authority of Faraday has made this, ever since, to be the generally-accepted explanation of the phenomena. It may seem presumptuous to question it, but I cannot think that a careful examination of his experiments will justify all of his conclusions.

Faraday's apparatus consisted of a small steam-boiler, which he insulated, and for the discharge of steam he attached a pipe about four feet long, terminating in an iron globe. This had an orifice to which other appendages could be attached, and there was also a device for injecting water into the exit-pipe. His first experiments were directed to evaporation. He found that when the steam was at full pressure, and the valve was suddenly raised and taken out, and the evaporation was very rapid, no electricity was produced. He charged the boiler with electricity by an electrical machine before the valve was raised, and found that the escape of steam did not affect the charge. It was hence inferred that no electricity was produced by evaporation.

The circumstances under which electricity was produced were the next subjects of his inquiry, and he says (section 2084):

"The issue of steam alone was not sufficient to evolve electricity. Attaching appendages to the globe, with no water in it, after a few moments and when the apparatus became hot, the issuing steam excited no electricity; but when the steam-globe was filled up with water so far that the rest of the condensed water was swept forward with the steam, abundance of electricity appeared. If then the globe was emptied of its water, the electricity ceased; but, on filling it up again to the proper height, it immediately reappeared in full force. So when the feeder-apparatus was used, while there was no water in the passage, there was no electricity; but, on letting in water from the feeder, electricity was immediately evolved."

It thus appears that, without water to condense the steam, there was no electricity, but with condensation an abundance appeared. Other experiments were to the same effect.

Faraday further says (section 2089):

"If there be no water in the steam-globe upon opening the steam-cock, the first effect is very striking: a good excitement of electricity takes place, but it very soon ceases. This is due to the water condensed in the cold passages producing excitement by rubbing against them. Thus, if the passage be a stopcock, while cold it excites electricity with what is supposed to be steam only; but as soon as it is hot the electricity ceases to be evolved; if then, while the steam is issuing, the cock be cooled by an insulated jet of water, it resumes its power. If, on the other hand, it be made hot by a spirit-lamp before the steam be let on, then there is no first effect. On this principle I have made an exciting passage by surrounding one part of the exit-tube with a little cistern, and putting spirits of wine or water into it."

Experiments with air were also made. It was compressed within a receiver and allowed to escape, impinging against ice or cones of wood or brass (section 2129). With common undried air electricity was produced. He says:

"This I attributed to the particles of water suddenly condensed from the expanding and cooled air rubbing against the metal or wood. Such particles were very visible in the mist that appeared, and also by their effect of moistening the surface of the wood and metal. . . . I proceeded to experiment with dry air (artificially dried by absorbents), and found that it was in all cases quite incapable of exciting electricity against wood or sulphur or brass in the form of cones; yet if in the midst of these experiments I let out a portion of air immediately after its compression, allowing it no time to dry, then it rendered the rubbed wood or brass negative. This is to me a satisfactory proof that in the former case the effect was due to the condensed water, and that neither air alone nor steam alone can excite these bodies, wood, brass, etc., so as to produce the effect now under investigation."

Under all circumstances, then, condensation produced electricity. Without it there was none. The theory of friction is wholly unnecessary to the explanation of the phenomena. It has to assume that water, a conductor, rubbing against iron, another conductor, will accumulate electricity. But this is opposed to our experience. It requires friction with a non-conductor to excite electricity. We have instances daily of the passage of water through pipes, sometimes with great force and velocity. Did friction between the two excite electricity, it should be produced in great quantities. But no such effect has been observed.

Mr. Patterson, who experimented with the same boiler that Armstrong used, tried the effect of blowing out water instead of steam through a pipe from the boiler, and no electricity was thereby produced (Philosophical Magazine, vol. xvii., p. 459, 1840).

Were it indeed true, as Faraday assumed, that the friction of water against the sides of the exit-pipe produced electricity, it would be conducted away as fast as formed by the metallic tube to the negative boiler. Indeed, when he placed in the tube any saline or acid substance that increased the conducting power of water, no electrical effects were obtained. In some cases negative electricity, like that of the boiler, was manifested.

I do not find, from an examination of Faraday's paper, that he made any experiment upon steam at a distance from its exit, where its condensation to water mostly took place. His mode of experimenting he describes as follows. He says (section 2082):

"When the issuing steam produces electricity, there are two ways of examining the effect. Either the insulated boiler may be observed or the steam examined; but these states are always contrary one to the other. . . . To examine the state of the boiler or substance against which the steam is excited, is far more convenient, as Mr. Armstrong has observed, than to go for the electricity to the steam itself. And in this paper I shall give the state of the former, unless it be otherwise expressed."

I infer, therefore, that, in all the experiments hereinbefore detailed, the tests for electricity were made at or near the boiler, and the very important inquiry of the electrical effects at a distance, where condensation alone was concerned, seems not to have attracted his attention. This point, however, has been fully investigated by others.

Mr. Patterson attached ten or twelve pointed wires to a copper rod. These were bent downward and held in the escaping steam. He says (ibid., page 458):

"The sparks were larger when the points of the conductor were held in the steam about two feet above the valve; but large sparks were obtained by holding the conductor entirely out of the cloud of steam and at a distance from it, for the air in the wooden shed in which we operated became speedily electrical throughout. The electricity was positive."

Mr. Armstrong also states (ibid., page 453):

"Upon trying the steam in the first instance by the method adopted in the previous cases, that is to say, by standing upon an insulated stool and holding with one hand a light iron rod immediately above the safety-valve while the steam was freely escaping, and then advancing the other hand toward any conducting body, sparks of about an inch in length were obtained. But it was soon observed that, by elevating the rod in the steam, the electricity was gradually increased, and that the maximum effect was not obtained until the end of the rod was raised five or six feet above the valve, at which point the length of the sparks occasionally reached two inches. Small sparks were even obtained when the rod was wholly removed from the steam and held in the atmosphere at the distance of two or three feet from the jet; and the electricity thus drawn from the air was positive like that of the steam. When the rod was extended into the cloud of vapor which accumulated in the upper part of the shed, electricity was drawn down as by a lightning-conductor from a thunder-cloud."

These results seem to me to point out very clearly the cause of the electrical excitement. If it were the friction of steam against the sides of the exit-pipe, then at that point should be the greatest manifestation. If, on the contrary, it be condensation, then at a distance from the exit, where the greatest condensation takes place, should be the greatest development—and such is the fact. At the valve, Faraday found no electricity. At five or six feet from it, according to Armstrong and to Patterson, its development was abundant. The conclusion seems to me inevitable. All the facts point to condensation as the cause of the excitement.

A phenomenon that occurs daily at Pike's Peak has been described to me. The tops of the mountain, are covered with perpetual snow. During the summer months the snow-line gradually recedes up the sides of the mountain, and from it flow considerable streams of water, that are finally lost in the plains below. The winds coming from the prairies take up this moisture, and, ascending the mountain, reach the frozen regions above. At about eleven o'clock each day, black clouds begin to gather about the tops of the mountain, and soon thereafter pour down floods of rain. Flashes of lightning are almost incessant, with peals of thunder that seem to shake the mountains. I am informed that we have nothing in Eastern States that can compare with the terrific violence of these storms.

Here, certainly, is no friction, but condensation on a large scale; and it is attended with the same electrical effects that were observed in the condensation of steam from the steam-boiler.

Volcanoes sometimes emit great volumes of steam and smoke, and these are usually attended with flashes of lightning in every direction. Were the electricity due to friction, it would be found at the mouth of the crater, where the steam issues; but, instead of that, it is found on the sides of the column, where the steam meets with colder air, and is condensed to water. The effects are analogous in every respect to those of steam from the steam-boiler.

In thunder-storms we have no friction, but condensation, and we need not go beyond the usual effects of condensation to explain all the electrical phenomena.

1. Paper read before the Philosophical Society of Washington.
2. "Researches in Electricity," vol. ii., section 2085.