As regards the general phenomena of atmos pheric electricity, observations throughout the globe harmonize in showing that, in general, in calm, clear weather a difference of potential ex ists such that the air is positive and the earth negative ; this difference is larger during east and northeast winds in the Northern Hemis phere, and it oscillates violently during thunder storms; there are also two daily maxima and two minima ; the potential difference is much larger in winter than in summer, except during thunder storms; the difference increases greatly during falling snow and during strong winds. Lord Kel vin suggests that there may be cloudless masses of air having different charges of electricity float ing above and producing the observed changes in potential as they pass by the observer.
As regards the origin of atmospheric electric ity, a great number of hypotheses have been ad vanced and discussed ; most of the older ones have been shown to be unsatisfactory, but the newest suggestions are still discus sion, and our knowledge of the whole subject re mains in an elementary condition. Perhaps the nearest approach to a plausible explanation is found in the work of J. J. Thomson and C. T. R. Wilson, who have been able to show that when aqueous vapor begins to condense from the air it settles by preference on the particles of dust that have a negative electrical charge and that a con siderable degree of supersaturation is required to make it condense upon those that are positively electrified. It follows that the first formed par ticles of fog or cloud are negative and are larger and, therefore, heavier ; and that they, settling to the earth as rain, give the ground and the lower portion of the atmosphere a negative charge rel ative to the upper strata of air. This explana tion, is, perhaps, better than another which has had many adherents, namely, that the earth, be ing negatively electrified, induces a positive charge in the air. The action of ultra violet radiations in discharging a negatively electrified body is the same as though they them selves were positively electrified ; and this has led to the hypothesis that the radiation from the sun may give a positive charge to the upper layers of the atmosphere. The hypo
theses that atmospheric electricity is due to the evaporation of water at the earth's surface, or to the friction of the wind, or to thermoelectric cur rents, or to the discharge of great volumes of steam through volcanic vents and geysers, must all be given up as insufficient. If it should be found that we must return to the original theory of Peltier that the earth has a negative charge, and that the atmosphere becomes positive by in duction, then the most plausible suggestion—but as yet undemonstrated—is that of Clerk Max well, namely, that the tidal strains within the earth's crust and the slipping that cause faults and earthquakes give rise to a development of piezoelectricity that has gradually accumulated until the present condition has been attained. This hypothesis connects terrestrial electricity and terrestrial magnetism together, and may be considered as a new example of the transforma tion of energy, since it is equivalent to the daily conversion of a small fraction of the force of gravitation into electricity.
For a general summary of our knowledge of atmospheric electricity at the present time, see Hann, Lchrbuch der Meteorologic (Leipzig, 1901), pages 711-24. For special memoirs, see Mendenhall, "Report on Atmospheric Electricity," Memoirs of the National Academy (Washington, 1889) ; Exner, Beitriige zur Erkcntniss der at mosphiirisehen Elektrizitiit, a series of memoirs in the successive volumes of the Sitzungsberiehte of the Vienna Academy of Sciences; J. J. Thom son and C. T. R. Wilson, numerous articles in the Proceedings of the Philosophical Society of London; Elster and Geitel, numerous memoirs in the Sitzungsberiehte of the Viemm Academy of Sciences; Elster, a general report published in Terrestrial Magnetism (Baltimore, 1900) ; and also in the Memoirs of the International Congress of Physics (Paris, 1900).