Electricity, ATMOSPHERIC. That the atmosphere, occasionally at least, is in a state of electrification is readily shown by the well-known phenomena of thunder-storms, and that the electrical conditions found on such occasions are the same as that obtained in any laboratory experiment with electrical apparatus was proved by Franklin in 1752 by his famous kite experiment. But since then it has been shown by numerous observations that the atmosphere is constantly in a state of greater or less electrification; that it is sensibly electrified not only during thunder-storms, but also during the prevalence of calm, clear weather. The observations made on the electrical condition of the atmosphere have mainly been the determination of the potential (see ELECTRICITY) of the earth's surface at a given time and place, or of the difference between the potential at a point in the air and that at the nearest point on the earth's surface. The distinction between these two determinations may be made clear by an example. If, for instance, a spherical ball be placed on the top of a rod of conducting material in connection with the earth, and having been lifted from that position by an insulated handle or carrier, be taken to an electrometer, with which to measure its potential, we would get by the latter an estimate of the potential of the earth's surface at the place formerly occupied by the rod and ball. If, on the other hand, we imagine a conductor placed at a given point in the air to become insulated, and then to throw from its surface portions of its mass, it would soon be reduced in potential so as to be in electrical equilibrium with the surrounding air—i.e. it will assume the potential of the air at the point. By afterwards finding the potential of this conductor we obtain a determination of the potential of the air at the point, or, what comes to the same thing, of the difference between the potential of the earth and that of the air at the given point.
The first of these methods was employed by Delmann in an extensive series of observations, but it has been by the second that more accurate observations have recently been made. It is, moreover, capable of being so adapted that a continuous record by means of photography may be obtained. Sir William Thomson, in his experiments in the island of Arran, employed two forms of this second mode of experiment. In one he used a burning match or spirit flame on the end of a long vertical insulated conducting rod; the products of combustion continually carry off electricity so long as the potential of the rod is different from that of the air surrounding the flame. The potential of the conductor is thus quickly brought to be the same as the air at the flame. To the lower end of the rod a small portable electrometer is usually attached, from the readings of which the difference between the potential of the earth and that of the air at the flame may be deduced. The other method depends on the same principle. It consists in insulating a can of water which has a long tube projecting from the lower part of its side. The can is usually placed near to and inside the window of a room, while the projecting tube passes through the window. Water is allowed to trickle slowly from the end of the tube at a nozzle, usually a few feet from the window-sill. As each drop falls it carries with it a small charge of electricity, and this goes on until the potential of the can is reduced to that of the air at the nozzle of the tube; this potential is afterwards found by means of the electrometer.
By such means Sir W. Thomson found that the difference of potential for a point 9 feet from the ground was equivalent to that of from 200 to 400 Daniell's cells. This represents a resultant force perpendicular to the earth's surface of from about 22 to 44 Daniell's cells per foot. He also found that the electrical conditions were subject to much variation, due no doubt to the motion overhead of electrified cloud-masses, at no great distance from the ground. But generally in calm clear weather the electrometer readings indicated a high positive potential, increasing so markedly before and during east and north-east winds as to be of some value in forecasting the weather. Other observations, chiefly in Europe, show that the potential of the air reaches two maxima and two minima each day, and that the hours of these maxima and minima are different for different seasons of the year. For instance, at Brussels, the two maxima in summer were found to be at 8 A.M. and 9 P.M.; in winter, at 10 A.M. and 6 P.M.
And to the causes of atmospheric electricity, many theories have been propounded, but most of them assume to begin with either some small initial charge of electricity in each vapour particle in a cloud, or some charge of electricity more or less permanent, on the earth's surface. We have only space to mention one or two of the more notable speculations. One which has held a place for some time past is that in which the evaporation going on at the surface of the sea and lakes is made the origin of atmospheric electricity—that in the formation of vapour the particles assume an opposite charge to the mass of liquid which they leave. But it has recently been shown that a much higher temperature is required for such electrification than that which ordinarily is found in large masses of water on the earth's surface. Again, in the condensation of vapour has been sought the source of the phenomena; but that it should be so is difficult to believe, for the very reasons which make the previous speculation somewhat plausible. Some physicists have assumed that the earth has a permanent negative electric charge, and that clouds coming in contact with mountains become charged. That this may be so in special localities is highly probable, yet the original earth's charge is not explained. Sir W. Thomson assumes to begin with, that the lower layers of the atmosphere have a permanent positive charge, which is diffused by convection currents into the upper layers. Atmospheric electricity has also been set down as the result of the friction of dry air against moist air; or, again, by some to the friction of the air in passing over the earth's surface. But this theory is discredited by the general observation that thunder-storms are more frequent in calm than in windy weather.
The true cause of atmospheric electricity has therefore not yet been discovered. But if any theory can rationally explain a mode by which each small vapour particle in a cloud may become electrified, even with an excessively small charge, the main difficulty would be got over, and the rest would be comparatively easy. The potential of each spherical particle is directly proportional to the quantity of electricity with which it is charged; and when several spheres unite together to form one sphere, the potential of the single sphere is proportional to its surface, compared with the smaller particles of which it is composed. When it is remembered that the number of particles in a drop of water is reckoned at many thousands of billions, it is easy to conceive how a thunder-cloud may have a very high potential, even although the particles may originally have had an extremely small charge.
Meanwhile, it would appear that a widely extended series of observations on the electrical conditions of the atmosphere, together with experiments on a much larger scale than is usual in a laboratory, are necessary requisites before much more light can be thrown on the subject. See LIGHTNING, THUNDER.