Atmospheric pollution and potential gradient at Kew observatory, 1921 and 1922

Abstract

1. It has long been known that fog at Kew Observatory is usually associated with high values of the potential gradient of atmospheric electricity, and it was found some years ago that the influence was not confined to thick fog, but prevailed to a greater or less extent in less opaque conditions of the atmosphere, diminishing as the opacity diminished. The introduction of one of Dr. Owens' pollution recorders has enabled a more complete investigation to be made of the influence of impurity of the atmosphere on the potential gradient. The pollution and potential records for two complete years, 1921 and 1922, are utilised. Before proceeding further, a reference is necessary to the nature of the pollution records. The amount of impurity is determined by the blackening of a paper filter, through which a definite volume of atmospheric air has been drawn. The blackened area is compared optically with standard papers, and a pollution number 0, 1, 2, etc., is assigned accordingly. Two points call for particular notice. There is necessarily a considerable interval of pollution covered by any one pollution number, and in particular No. 0 does not really mean a total absence of pollution. The second point is that the assumption that a given darkening of the biter paper answers to a definite mass of dirt per unit volume of the atmosphere may be only an approximation to the truth. Conceivably, some dirt may, mass for mass, blacken the biter paper less than other dirt. The composition of the dirt in the atmosphere may vary throughout the year. There may, for instance, be differences in the sizes of dust particles, and the influence on the potential gradient may depend on the average size of the particle, as well as on the dirt mass per cubic metre. minute knowledge of the composition of the dust may be essential to a complete study of its action. On the other hand, as will appear presently, the effect is not a small one, and it has appeared desirable to publish the results obtained without further delay.