378 Mr Wilson, On the Measurement of the Earth-Air 



It is possible to test by a slight modification of the conditions 

 of the experiments just described a whole class of theories of the 

 origin of atmospheric electricity ; those namely which suppose 

 the electric field in fine weather regions to be maintained or 

 regenerated by the action of the air on earth-connected bodies 

 in these regions. Such theories would imply that air in passing 

 over the earth's surface becomes positively charged by friction, by 

 a difference in the mobilities of the positive and negative ions, or 

 otherwise. Let the usual observations of the charge on the test- 

 plate, or on a plant or other body placed upon it, while the potential 

 is kept at zero, and of the charge gained by the system under 

 these conditions in a given time, be made under the shelter of a 

 tree. Under these conditions the electric field and the charge on 

 the body under test will almost vanish, so that there is nothing to 

 prevent the negative electrification of the body, which according 

 to such theories would result. 



Experiments of this kind have been tried both with the 

 ordinary brass test-plate and with a plant. In no case was there 

 any evidence of any tendency for a negative charge to be acquired, 

 the current remaining in the normal direction even in exceedingly 

 weak fields. 



Note on the Origin of Atmospheric Electricity. 



I take this opportunity of modifying an opinion expressed in an 

 article in Nature (June, 1903), in which I discussed some possible 

 sources of atmospheric electricity. I there spoke of the difficulty 

 which the recently discovered conducting power of the air intro- 

 duced into the theories which explain the fine weather electric 

 field by the action of precipitation in wet weather regions. 

 Assuming the electrical current, which according to this view must 

 travel in the atmosphere from wet weather to fine weather regions, 

 to be entirely one of convection, the positive charge being carried 

 along by upper air currents, I pointed out that the greater part of 

 the positive charge of the air would have disappeared by conduc- 

 tion to the earth before the air had travelled any considerable 

 distance. Thus it would be impossible to explain the existence of 

 an electric field at great distances from areas of precipitation. I 

 therefore suggested that we might be driven to look to a cosmical 

 source for the maintenance of the earth's charge, e.g. to negatively 

 charged particles of great penetrating power traversing our 

 atmosphere and being absorbed by the earth. 



The possibility of the current from the wet regions to the 

 regions of fine weather being, like the vertical electric current at 

 the surface, not due to convection but to conduction, i.e. to move- 

 ment of the free ions under the influence of an electric field, was 

 there ignored. For this I now think there is no justification. 



