﻿234 On the Electrification of Air, 



To particularize further, suppose the potential to have been 

 38 volts or 0*127 electrostatic C.G.S. (which is less than the 

 greatest found in our experiments) and take a = 50 centim. : 

 we find p = 2*4 x 10~ 5 . The electrostatic force at distance r 

 from the centre, being f Trpr, is therefore equal to 10" 4 ?'. 

 Hence a small body electrified with a quantity of electricity 

 equal to that possessed by a cubic centimetre of the air, and 

 placed midway (r = 25) between the surface and centre of the 

 enclosure, experiences a force equal to 2'4 X 10 -9 25, or 

 6 x 10 -8 , or approximately 6 x 10~ 5 grammes weight. This is 

 4 - 8 per cent, of the force of gravity on a cubic centimetre of 

 air of density 1/800. 



§ 14. Hence we see that, on the supposition of electric 

 density uniform throughout the spherical enclosure, each 

 cubic centimetre of air experiences an electrostatic force 

 towards the boundary in simple proportion to distance from 

 the centre, and amounting at the boundary to nearly 10 per 

 cent, of the force of gravity upon it ; and electric forces of 

 not very dissimilar magnitudes must have acted on the air 

 electrified as it actually was in the non-spherical enclosure 

 used in our experiments. If natural air or cloud, close to the 

 ground or in the lower regions of the earth's atmosphere, is 

 ever, as in all probability it often is, electrified to as great a 

 degree of electric density as we have found it within our 

 experimental vat, the natural electrostatic force in the atmo- 

 sphere, due as it is, no doubt, to positive electricity in very 

 high regions, must exercise an important ponderomotive force 

 quite comparable in magnitude with that due to difference of 

 temperatures in different positions. 



It is interesting to remark that negatively electrified air 

 over negatively electrified ground, and with non-electrified 

 air above it, in an absolute calm, would be in unstable equili- 

 brium ; and the negatively electrified air would therefore rise, 

 probably in large masses, through the non-electrified air up 

 to the higher regions, where the positive electrification is 

 supposed to reside. Even with no stronger electrification 

 than that which we have had within our experimental vat, 

 the moving forces would be sufficient to produce instability 

 comparable with that of air warmed by the ground and rising 

 through colder air above. 



§ 15. During a thunderstorm the electrification of air, or 

 of air and the watery spherules constituting cloud, need not 

 be enormously stronger than that found in our experiments. 

 This we see by considering that if a uniformly electrified globe 

 of a metre diameter produces a difference of potential of 

 38 volts between its surface and centre, a globe of a kilometre 



