Popular Science Monthly/Volume 18/February 1881/Atmospheric Electricity
By Professor H. S. CARHART.
FROM the earliest periods the flash of lightning and the peal of thunder have excited curiosity, stimulated awe, and inspired fear in man; and according to his mythological, religious, or poetic habit of mind has he regarded the latter as the bolt of Jove, the voice of God, or the conscious utterance of the heavens. The explanation of these appearances in the sky is most curious and fantastic, even after the introduction of the modern inductive method. In a "Compendious System of Natural Philosophy," by J. Rowning, M.A., London, 1744, we find the following: "As vapors exhaled from the surface of water are carried up into the atmosphere, in like manner the effluvia of solid bodies are continually ascending thither. Now, we find by experiment that there are several inflammable bodies which, being mixed together in due proportion, will kindle into flame by fermentation alone, without the help of any fiery particles. When, therefore, there happens to be a mixture of the effluvia of such bodies floating in the air, they ferment, kindle, and, flashing like gunpowder, occasion those explosions and streams of fire which we call thunder and lightning."
Ever since Franklin identified lightning with the electricity of the frictional machine, an inquiry has been prosecuted into the origin of atmospheric electricity. It has been ascribed to chemical action in vegetation, without any basis of proof; also with more reason to evaporation and to friction of solid and liquid particles against bodies on the surface of the earth. But friction fails to account for the fact that electrical displays, other than auroral ones, occur only during heavy precipitation of hail or rain, except in mountainous districts.
Deferring for the present what appears to be a probable account of the generation of atmospheric electricity, we may profitably review certain facts established by observation and experiment.
It is a matter of common observation that a small ascending jet of water is resolved into drops, which describe widely divergent trajectories. By reason of the different velocities and directions of motion of the individual drops, they come into frequent collision with one another and then rebound. The influence of electricity on the recoil of the drops after collision is most marked and interesting. About two years ago, Lord Rayleigh read a paper before the Royal Society on this subject, setting forth the results of his experiments.
When the ascending jet is strongly electrified, the drops do not collide, because of their mutual repulsion, arising from their charge of electricity of like sign. But with a very feeble charge the drops coalesce upon impact, and the breaking up of the stream is thereby much lessened. The rubbing of a glass rod across the sleeve in the vicinity of the jet suffices to prevent the rebound after collision; as also does the current from a single Grove element.
Further experiments, in which one of two contiguous jets was electrified, proved that the coalescence was due to a slightly different degree of electric tension in the impinging particles of water. In such case their attraction and coalescence are determined by static induction, the resulting force of which is always attractive; and with slight electrification the inductive effect always prevails over the repulsion due to like charges, when the charged bodies are brought near together. Hence strongly electrified particles do not actually collide, but are kept apart by electrical repulsion; while those feebly charged approach within the charmed circle where the attraction of static induction determines their collision and coalescence.
The bearing of these facts upon precipitation of aqueous vapor is obvious. Innumerable globules of water, feebly charged to different potentials, inevitably collide and coalesce into drops, which descend by gravity. A slight amount of electricity in the atmosphere is therefore favorable to aqueous precipitation, while a higher degree of electrical excitation is unfavorable to rapid condensation.
It is important in this connection to point out another conclusion bearing upon condensation of vapor—a conclusion reached by applying mathematics to the theory of electrified spheres or drops of water. An explanation of terms is essential to an understanding of the reasoning.
We have already employed the term "potential," a word of frequent occurrence in modern electrical works. An insulated, charged conductor exerts influence in every direction about it. The term "electrical potential" expresses the value of that influence at any specified point where there is electrical force. When any force acts, energy is expended in doing work; and the numerical value of the scientific expression "work" is the measure of the energy expended or the resistance overcome. "Work" means the product of the force into the distance over which it acts. The lifting of five pounds through a distance of ten feet requires the expenditure of fifty foot-pounds of work. Hence "electrical potential" may be defined in terms of work as follows: The electrical potential at any point is the work required to carry a unit of electricity from that point to infinity. It is of course understood that the "unit of electricity" is carried against the attraction due to an electrified body.
Potential is accordingly a mathematical and exact expression for all the work possible to be done against the attraction of a given amount of electricity resident at some specified place. In exactly the same way gravitational potential at any point may be defined as the work required to carry a unit mass of matter from that point to infinity against the attraction of gravitation.
Electrometers have been constructed by Sir William Thomson, with marvelous skill and inventive genius, designed to measure the potential at any point or of any body. It is susceptible of easy proof that the potential of a sphere, charged with electricity, Q, is everywhere equal to Q divided by its radius R. Further, the capacity of a body for electricity is the quantity required to charge it to emit potential; and, as potential varies with the charge, we readily find that the capacity of a sphere is numerically equal to its radius.
To apply this to condensation, suppose that drops of water of unit radius, unit capacity, and unit potential, coalesce to form drops of radius two: what will be the capacity and potential of such larger drops? Since the volume of spheres varies as the cube of their radii, eight of the small drops will be required to make one large one. The large drop will, therefore, contain eight times as much electricity as each of the small ones. As compared with the small, drops, its capacity, being equal to its radius, will be only doubled, while its charge will be increased eightfold; and its potential will, therefore, be four times as great as that of the small drops, since potential of a sphere equals quantity of electricity divided by capacity of sphere. If its potential is quadrupled, its inductive influence on other bodies and its tendency to discharge are increased in the same ratio.
When, therefore, condensation occurs, aggregating minute globules of water into larger ones, the electric tension of the mass of descending vapor is immensely increased, without any corresponding increase in the total quantity of electricity present.
These two conclusions, applicable to condensation, may be applied to the frequently observed fact that a vivid flash of lightning is often quickly followed by a sudden and heavy down-pouring of rain. It is clearly impossible to tell which is antecedent to the other, the discharge or the sudden condensation; for, while the flash reaches the observer first, it is clear that light travels from the place of action with a vastly greater velocity than that of the falling rain, and the discharge may therefore have been subsequent to the sudden condensation. If the discharge occurs first, then the lowering of the electric potential permits approach of aqueous spherules and consequent coalescence after collision, in accordance with Lord Rayleigh's experiments. On the other hand, if the condensation is antecedent, it follows from the result reached above that it must be accompanied by a sudden rise of potential in the enlarged drops, leading to an electric discharge.
It will be observed that neither of these principles accounts for the original electrification of aqueous vapor in the air. It has been the custom to regard thunder-clouds as primarily charged by some unexplained process; and these, acting inductively, as producing a corresponding charge of opposite sign in the earth underneath. This view appears to have no conclusive evidence in its favor, but corresponds rather to appearances merely—a very unreliable guide.
In the theory here proposed, the earth is the charged body, acting inductively on the air, aqueous vapor, and clouds about it. Whenever moisture condenses to cloud, a better conductor is thereby formed, and increased inductive action takes place, causing an accumulation of electricity both in the cloud above and the earth beneath. If, then, the lower part of the cloud, under this inductive influence, condenses to rain and falls away from the upper part, a separation of the two electricities is effected, and an increase in potential results from the enlargement of the drops, as explained above. Consequently, a discharge may then take place either between the upper and lower cloudy strata, or between the lower portion and the earth, according as one path or the other offers the least resistance.
Further, when evaporation takes place from an electrified locality, the rising vapor must carry away a charge of electricity by convection, as air and dust carry away electricity from a charged conductor. The condensation of this vapor increases its potential, and, if sufficiently rapid, gives rise to electrical displays. It is a fact of recent establishment that "northern lights" occur in various high latitudes only with southerly winds, which come laden with moisture and probably with electricity.
The following considerations in favor of this view of the origin of atmospheric electricity may be briefly enumerated:
1. Continuous observations of the electrical state of the atmosphere at Kew Observatory and elsewhere for several years show that the air is always more or less electrified. The average potential of fair weather being 4, it was found rarely to fall as low as 1; often during sudden showers it equals 20 or 30; during snowstorms with high wind it sometimes reaches 100; and during thunder-showers, 100 and even 200. A predominance of negative electricity is characteristic of thunderstorms. Hence some origin of atmospheric electricity, always operative, but in different degrees of intensity, must be sought. Clearly, any form of energy connected with mere cloud-formation will not answer the requirements.
2. The observations show also that the potential of the air is exceedingly fluctuating, no natural phenomenon being comparable with it in changeableness except wind-pressure. A change in the electrical state of the air indicates a corresponding change in the earth's surface. It is more reasonable to suppose that the earth, the great reservoir of electricity, should control the air and clouds electrically than that the clouds should control the earth.
3. The surface of the earth is perhaps never in electrical equilibrium; in other words, it is not an equipotential surface. Some time about 1865 Matteucci made prolonged experiments on earth-currents, and reached several interesting results. He found, for instance, that a tolerably steady current of electricity flowed through a line established along a meridian, uniformly from south to north; that fluctuating currents of low electromotive force flowed through an east and west line, sometimes in one direction, sometimes in the other; that when one terminal of a long line was in a valley and the other at a considerably higher elevation, a current flowed uniformly up the wire toward the more elevated end. A flash of lightning, in this case, was always accompanied by a sudden increase in the deflection of the galvanometer needle. ("Smithsonian Reports," 1867.)
4. Irregular and spontaneous earth-currents are the usual accompaniment of great terrestrial disturbances. James Graves showed in 1871 that spontaneous currents in the Atlantic cables frequently occur during earthquake-shocks. ("Journal Soc. Tel. Eng.," ii, pp. 80-120.) That spontaneous currents flow through land lines during auroral displays is a well-known fact. It is also asserted that any great meteorological change, as the motion of a heavy storm with considerable barometric fluctuation, is announced at a distance by irregular galvanic shocks through submarine cables.
5. Marked electrical disturbances in the atmosphere not infrequently accompany earthquakes and volcanic eruptions. A vivid flash of lightning was seen during an earthquake-shock in West Cumberland, England, on October 25, 1879. So frequently are these two phenomena conjoined that some writers have attributed South American earthquakes to electrical action. Lightning is often seen playing about the boundary between the condensing vapor from a volcano and the adjacent cool air.
6. Measurements of the potential of the air show that, as we proceed farther from the surface of the earth, the potential of points in the air differs more and more from that of the earth, the difference being approximately simply proportional to the distance. Also the electrical density is greater on projecting parts of the earth's surface than on those which are plane or concave. These facts are precisely what we should find to be the case in the vicinity of an irregular, charged conductor; and they are sufficiently explained if we regard the earth itself as charged with electricity varying in density. In fact, observations of the potential of so-called atmospheric electricity are simply "determinations of the quantity of electricity residing on the earth's surface at the place of observation." (Professor Everett, in Deschanel's "Natural Philosophy.")
In the Rocky Mountains electrical storms are of frequent occurrence. They consist of electrical displays without precipitation of rain, hail, or snow. Usually, though not always, the sky is overcast. In February, 1880, a remarkable electrical excitation was manifest at Boulder, Colorado. The miners were unable to kindle fire in the stove till eleven o'clock in the morning, every attempt to touch the metal about the stove resulting in a severe electrical shock. With every strong gust of wind the manifestations were more marked. Similar reliable evidence comes to the writer from other parts of Colorado. Long's Peak, an isolated mountain, 14,271 feet high, is noted for these peculiar electrical manifestations. The density of electricity on the peaks, projecting so far above the general level of the earth's surface, is greater than elsewhere; hence they possess a power to discharge like points on an electrified conductor. The air and aqueous vapor become surcharged with electricity; and only a slight condensation, sufficient merely to form clouds without rain, serves to produce discharges of lightning. With heavy gusts of wind the charged air is removed, and a fresh supply is provided into which the peak again pours its electricity.
In view of these facts, the theory is submitted as worthy of consideration that the earth itself is the seat of those disturbances that manifest themselves in atmospheric electricity. Fluctuating currents ebb and flow through the confining walls of this immense reservoir of cosmic energy. These follow naturally from the great changes in temperature to which the earth's crust is subjected; from those seismic disturbances occasioned by vast internal convulsions; from immeasurable local strain and compression, the result of upheaval and contraction. The earth, unlike the moon, contains still a vast store of unexpended energy; and in the ebb and flux of its mighty internal, contending forces, and the bending and swaying of its magnetic lines of force, in obedience to the magic wand of the sun, there is ample room for the generation of those comparatively feeble forms of energy that manifest themselves in the electrical disturbances of the air.