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XXIV. On the Electrification of Air. By Lord Kelvin, 

 P.B.S., and Magnus Maclean, M.A., F.R.S.E.* 



§ 1. f I lHAT air can be electrified either positively or nega- 

 I tively is obvious from the fact that an isolated 

 spherule of pure water, electrified either positively or nega- 

 tively, can be wholly evaporated in airf. Thirty-four years 

 ago it was pointed out by one of us+ as probable that, in 

 ordinary natural atmospheric conditions, the air for some 

 considerable height above the earth's surface is electrified § 



* Communicated by the Authors ; having been read before the Royal 

 Society, May 31, 1894. 



t This demonstrates an affirmative answer to the question, Can a 

 molecule of a gas be charged with electricity ? (J. J. Thomson, i Recent 

 Researches in Electricity and Magnetism,' § 36, p. 53), and shows that 

 the experiments referred to as pointing to the opposite conclusion are to 

 be explained otherwise. 



Since this was written we find, in the 'Electrical Review' of May 18, 

 on page 571, in a lecture by Elihu Thomson, the following : — " It is 

 known that as we leave the surface of the earth and rise in the air there is 



an increase of positive potential with respect to the ground It is 



not clearly proven that a pure gas, rarefied or not, can receive and convey 

 a charge. If we imagine a charged drop of water suspended in air and 

 evaporating, it follows that, unless the charge be carried off in the vapour, 

 the potential of the drop would rise steadily as its surface diminished, 

 and would become infinite as the drop disappeared, unless the charge 

 were dissipated before the complete drying up of the drop by dispersion 

 of the drop itself, or conveyance of electricity by its vapour. The charge 

 would certainly require to pass somewhere, and might leave the air and 

 vapour charged." 



It is quite clear that "must" ought to be substituted for "might" in 

 this last line. Thus the vagueness and doubts expressed in the first part 

 of the quoted statement are annulled by the last three sentences of it. 



% " Even in fair weather the intensity of the electric force in the air 

 near the earth's surface is perpetually fluctuating. The speaker had often 

 observed it, especially during calms or very light breezes from the east, 

 varying from 40 Daniell's elements per foot to three or four times that 

 amount during a few minutes, and returning again as rapidly to the lower 

 amount. More frequently he had observed variations from about 30 to 

 about 40, and back again, recurring in uncertain periods of perhaps about 

 two minutes. These gradual variations cannot but be produced by elec- 

 trified masses of air or cloud, floating by the locality of observation." — 

 Lord Kelvin's ' Electrostatics and Magnetism,' art. xvi. § 282. 



§ " The out-of-doors air potential, as tested by a portable electrometer 

 in an open place, or even by a water-dropping nozzle outside, two or 

 three feet from the walls of the lecture-room, was generally on these 

 occasions positive, and the earth's surface itself therefore, of course, nega- 

 tive — the common fair-weather condition — which I am forced to conclude 

 is due to a paramount influence of positive electricity in higher regions of 

 the air, notwithstanding the negative electricity of the air in the lower 



Phil. Mag. S. 5. Vol. 38. No. 231. Aug. 1894 Q 



