Current and on the Origin of Atmospheric Electricity. 381 



have a potential fall in rising from the point considered into the 

 conducting upper layers only a few hundred thousand volts less 

 than in travelling from the point to the earth. There will thus be 

 a flow of positive electricity into the upper atmosphere, helping to 

 maintain its positive potential. Travelling with the current from 

 the source we should have a very large fall of potential in rising 

 to the conducting layers, a comparatively small fall in travelling 

 laterally in the conducting layers, and a fall of a few hundred 

 thousand volts in passing through the lowest few thousand metres 

 to the ground. 



It is a matter of fundamental importance to determine whether 

 the negative charge carried down by rain is sufficient to counter- 

 balance the conduction current from the atmosphere to the earth's 

 surface over the whole world. We are of course a very long way 

 from the solution of this question; according to Gerdien* however, 

 to whom with Elster and Geitel what knowledge we possess as to 

 the electrical charges brought down by precipitation is mainly due, 

 there is at least agreement in the order of magnitude of the 

 quantities. 



The variations of the potential gradient near the ground, e.g. 

 the daily variations, are mainly to be attributed to variations in 

 the conductivity of the different parts of the atmosphere. The 

 increase in the potential gradient due to fog or haze near the 

 ground has long been known, and the explanation was given by 

 Elster and Geitel in one of the first papers in which our knowledge 

 of the existence of free ions in the atmosphere was applied to the 

 explanation of the phenomena of atmospheric electricity. It has 

 been pointed out by 0. W. Richardson (Nature, April 26, 1906) 

 that an increase in the conductivity at a height ought, according 

 to the view that I have been supporting here, to cause also an 

 increase in the potential gradient at the ground. For an increase 

 in the conductivity above will increase the current reaching the 

 surface of the earth, and if the conductivity of the lower layers is 

 unaltered the potential gradient in the lower layers must be pro- 

 portionately increased when a steady condition is established. A 

 change of conductivity of the lowest layers extending to only a few 

 metres above the ground will not alter to any considerable extent 

 the earth-air current in that region, and the potential gradient 

 when a steady condition is reached will be in this case inversely 

 proportional to the conductivity. 



Experiments have recently been described by Campbell and 

 Wood showing a daily variation of the ionisation within a closed 

 vessel resembling that of the atmospheric potential gradient, 

 showing for example an early morning minimum. l)r Richardson 

 has pointed out that if these results arc interpreted as indicating 

 • Genlicu, 1'ltyxi leal incite Zcitsclir. Jahrg. vi. 190-5. 



