Atmospheric Electricity and Fog


IN view of the interest recently shown in the subject of the dispersion or prevention of fog, it may be opportune to direct attention to a recent remarkable example of an atmospheric electricity phenomenon which usually accompanies London fogs. I should first explain that the method adopted at Kew for determining the absolute value of the potential gradient—i.e. the increase in the voltage per metre of height above the ground—certainly does not err in the direction of overestimating it. Taking eight years, 1898 to 1904, I found in a recent paper1 that the mean value of the potential gradient at Kew was 159, the mean value for January being 201. The phenomenon referred to above is the occurrence during fog of specially high positive potentials, values double or treble that appropriate to the season being not unusual. At such times, however, there are usually large and frequent oscillations in the value of the gradient, so that the maintenance of an exceptionally high value for a number of consecutive hours is comparatively rare. On the morning, however, of January 21, during an intensely thick fog, the potential, gradient at Kew exceeded 730 continuously from 1 to 9.30 a.m. How much it may have exceeded this value it is impossible to say, as the trace was beyond the limits of registration during the whole of this time. Both before the trace left the sheet and after its return the oscillations in the potential gradient were large, so that the maximum value was probably at least 1000.

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    Phil. Trans., A, vol. ccvi., p. 299.

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CHREE, C. Atmospheric Electricity and Fog . Nature 77, 343 (1908).

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