ATMOSPHERIC ELECTRICITY. Franklin was the first to establish the identity of the lightning of the heavens with the electric spark. By his famous kite-experiment, he ascertained that the thunder-cloud assumes an electrical condition precisely similar to that of the conductor of an electrical machine, and that the same mechanical and 111711i nous effects are though in a different degree, to both. The attention that was first directed by this discovery to the A. E., as displayed in the thunder-cloud, has since then been extended to the electrical condition of the air in all the different states of the weather. It is now found that the air is sensibly electrical not only when the sky is overcast with thunder-clouds, but when the weather is clear, or when no thimder-clonds are present. Observations on A. E. are made by delicate electrometers connected with insulated rods at the top of the building, or other collecting apparatus. The following arc some of the results got by continental observers: When the sky is clear and free from the A. E. is always positive, and an electroscope exposed to the action of the air is charged with positive electricity. On the other hand, the electricity of the ground is found to be negative. This was shown in a very ingenious way by Volta, who, by catching the fine spray of a fountain on the plate of a straw electroscope, found the straws to diverge with the negative electricity communicated to them by the water, which was necessarily of the same character as that of the ground. It is from this fact that electroscopes, or the collecting apparatus connected with them, must not be over topped by the neighboring trees or buildings, the negative electricity of which materially affects the indications given, and it is due to the same fact that no A. E. is discovered in the middle of a wood, or in a room, however high the ceiling. Ender a clear .sky, the potential of the X. E.. is found to increase as we ascend, the lower aOial strata being less electrical than the higher. Becquerel proved this by a simple experiment on the plateau of Mt. St. Bernard. On a piece of oiled silk he placed a silk thread, covered with tinsel, one end of which, terminated by a ring, was connected with the rod of a straw electroscope, and the other end was tied to an arrow armed with a metal point. When the arrow was shot horizontally, the straws showed no divergence; but when the arrow was shot upwards, they opened as it ascended, and diverged most when the arrow, in ascending., disengaged the ring from the rod of the electroscope. The same fact is shown in the following way; When a very delicate electroscope is adjusted for any particular position, it will, when elevated a few feet above that position, give indication of positive electricity, and when placed a few feet below, it will be charged negatively. In clear weather, likewise, the A. E. is found to be subject to certain daily periodical variations, and appears to have two maxima and two minima in the course of twenty four hours. The first maximum takes place a short time after sunrise, and the second shortly after sunset; the first minimum shortly before sunrise, and the second in the afternoon, when the heat of the day is greatest. In cloudy weather, the electroscope• is
affected sometimes positively, sometimes negatively, and is generally less influenced than in clear weather. The electricity of rain, snow, hail, etc., is sometimes positive, sometimes negative. In Stuttgart, for instance, it was found in the course of a year that the rain was 71 times positive to 60 times negative, and the snow 24 times positive to 6 times negative.
Sir William Thomson, in our own country, has made various observations on atmos pheric electricity. His delicate electrometers give him not only great facility of obser vation, but their delicacy far transcends that of any instrument hitherto employed in such observations. Instruments such as his electrometers, that are sensitive to the electromotive force of a single Daniell's cell with any condensing contrivance, are a wonderful advance in observing power. Sir William's collecting apparatus is an insulated can of water placed inside a window, with a nozzle extending four feet and a half beyond the wall, the window being only open so far as to admit of the nozzle-tube passing without touching. The can, when the stop-cock is opened, assumes the potential of the air outside at the point where the jet breaks up into drops. In the portable electrometer for outside observations, lie uses as the collector a burning match at the top of a long rod attached to the instrument. The collecting apparatus is, of course, insulated and connected with the electrometer. He estimates the amount of atmospheric electricity per foot or per inch. He calculates the difference of potential at the perpendicular distance, say, of a foot from any portion of the earth's surface, whether the level ground or an upright wall. He finds, as mentioned above, that the earth is always negative in clear weather, and the air positive, and that the difference of potential per foot is very different at different times. Thus, in the isle of Arran, he found this to vary in ordinary fine weather from 22 to 44 Daniell's cells; with an e. or n.e. wind, the difference of potentials was from 6 to 10 times that per foot. lie also finds sudden and unaccountable variations of potential within even comparatively few minutes, and he can only suggest that there may be cloudless yet cloud-like masses of clear air floating in the atmosphere, which are charged with electricity, and which, in their passage over or near the electrometer, give rise to these marked variations.
The cause of A. E. has given rise to much discussion. The electricity developed by evaporation and vegetation has been thought by some to account for the positive electricity of the air; but this view has been combated, and as yet no theory has been proposed which satisfactorily accounts for it. With the instrument that Sir William Thomson has placed in the hands of observers, and with a cordon of observers all over the world, data may be got for a satisfactory theory, but as yet our knowledge of the subject is too fragmentary to reach anything like a satisfactory account of it. For the electricity of the thunder-cloud, see LIGHTNING.