Mr Wilson, Measurement of the Earth- Air Current, etc. 363 



On the Measurement of the Earth-Air Current and on the 

 Origin of Atmospheric Electricity. By C. T. R. Wilson, M.A., 

 Sidney Sussex College. 



[Read 30 April 1906.] 



In ordinary fine weather there is, as is well known, an electric 

 field near the earth's surface, the direction of the field being down- 

 wards ; in other words, the surface of the ground is negatively 

 charged. We now know also that the atmospheric air contains 

 free ions, which will move under the action of the electric force. 

 Unless therefore there are other compensating processes at work, 

 there must be a continuous flow of positive electricity in fine 

 weather regions from the atmosphere into the ground. 



In order that we should be able to judge of the sufficiency or 

 otherwise of the various theories of atmospheric electricity, a know- 

 ledge of the magnitude of this current from the atmosphere into 

 the ground is required. For this current is a measure of the rate 

 at which the electric field is being destroyed, and therefore, if the 

 field remains undiminished, of the rate of regeneration of the 

 electric field that a theory of atmospheric electricity has to 

 account for. A knowledge of the earth-air current at various 

 places distributed over the earth's surface under different weather 

 conditions and at different times of the day and year is much to 

 be desired, especially if simultaneously measurements could also 

 be made of the charge carried down to the earth's surface by pre- 

 cipitation. Before however an attempt is made to carry out 

 systematically measurements of this kind, it is of importance that 

 suitable methods of making such measurements should be devised 

 and tested. 



The methods at present in use for deducing the value of the 

 earth-air current involve first a measurement of the potential 

 gradient at the time and place of observation, and secondly a 

 measurement of the leakage from a charged conductor under such 

 conditions that the current is proportional to the charge — in other 

 words, to the strength of the field at its surface. The latter 

 measurement, which is equivalent to a measurement of the con- 

 ductivity of the atmospheric air, is not an easy one to carry out 

 satisfactorily. The zerstreuungsapparat of Elster and Geitel, with 

 which the conducting power of the free atmospheric air was first 

 proved and its variations studied, does not give a reliable measure 

 of the true dissipation coefficient, the number of ions per c.c. in the 

 air under test being less than in the free atmosphere (except when 

 there is unusually efficient ventilation and rapid renewal of the 

 air by the wind) owing to recombination of the ions and their 



