2 So 



NATURE 



[July 19, 1S94 



THE ELECTRIFICA TION OF AIRy 

 % I. 'T'HAT air can be electrified either positively or nega- 

 lively is obvious from the fact that an isolated 

 spherule of pure water, electrified either positively or nega- 

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

 it was pointed out by one of as ' as probable that in ordinary 

 niiural atmospheric conditions, the air for some considerable 

 height above the earth's surface is elec- 

 trified,' and that the incessant variations 

 ' >f electrostatic force which he had observed, 

 minute after minute, during calms and 

 light winds, and often under a cloudless 

 sky, were due to motions of large quan- 

 tities of positively or negatively electrified 

 air in the immediate neighbourhood of 

 the place of observation. 



§ 2. It was proved ' by observations in 

 the Old College of Glasgow University 

 I hat the air was in general negatively 

 ■-lectrified, not only indoors, within the 

 • Id lecture room ■' of Natural Philosophy, 

 Imt also in the out-of doors space of the 

 College Court, open to the sky, though 

 closed around with high buildings, and 

 between it and the top of the College 

 Tower. The Old College was in a some- 

 what low situation, surrounded by a 

 densely-crowded part of a great city. 

 In the new University buildings, crown- 

 ing a hill on the western boundary of 

 Glasgow, similar phenomena, though with 

 less general prevalence of negative elec- 

 tricity in the air, have been observed, both 

 indoors, in tht large Bate Hall, and in 

 many other smaller rooms, and out-of- 

 door-, in the court, which is somewhat similar to the courts 

 of the Old College, but much larger. It is possible that 

 the negative electricity found thirty years ago in the air 

 of the Old College, may have been due to its situation, 

 surrounded by houses with their fires, and smoking factory 

 chimneys. In the New College much of the prevalence 

 of negative electricity in air within doors has, however, been 

 found to be due to electrification by the burning lamp" used 



' A Paper by Lord Kelvin, P.R.S., and Mr. Magnus Maclean, read at 

 ihe Royal Society on May 31. 



- This dern'jnstratcs an affirmative answer to the question, Can a molecule 

 °/* SV.*^ charg-d with electricity r (J. J. Thomson, " Recent Researchesin 

 Electricity and .Magnetism," § y.. p. 53) and shows that the experiments re- 

 ferred to as pointmR to the opposite conclusion arc tobe explainedotherwisc. 

 Since_ this was written, we find in the EUctrical Review of May 18. on 

 p. 571, in a lecture by Elihu Thomson, the fotlow>ng : — " It is known that 

 as we leave the surface of the earth and rise in the air. there is an increase 

 01 positive potential with respect to the ground. . . . It is not clearly proven 

 thai a pure gxs, rarefied or not, can receive and convey a ch.irge. If we 

 imagine a charged drop of water susficnded in air and evaporating, it f.»llows 

 that, unless the charge be carried off in the vapour, the potential of the drop 

 w luld rise steadily as its surface diminished, and would become infinite .as 

 the drop disappeared, unless the charge were dissipated before the complete 

 drying upof the drop by dispersion of the drop itself, or conveyanceof elec- 

 tricity by it« vapour. The charge would certainly require to pass somewhere, 

 and inight 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 

 "he quoted stairrnTnt are annulled by the Inst three sentences of it. 



■Mthcr the intensity of Ihe electric force in the air near 



l-erpclually fluctuating. The speaker had often ob- 



rir,^ ralms or ver>' light breezes from the east, vary- 



;. per f >ot to three or f.>ur times that amount 



'urning again as rapidly to the lower amount. 



■ 'ved variations from about 30 to about 40, and 



■ ;i 1. iccrtain i>cnods of perhaps about two minutes. 



us cinnol but be produce<l tiy electrified masse.s of 



: ^'.- the locality of observation." — Lord Kelvin's 



■i.m," art. xvi. ) 332. 



riiial, xs icstd by a portable electrometer in 



' tT^r ;r.,in.ing nozzle outside, two or threi feet 



''-rally on these occasions positive, 



r course, negative— the c-unmon 



• > I Mncludc istlue to .0 paramount 



•f the air, notwithstanding 



r iliim near the earth's sur* 



'or atmospheric cleclriciiy 



i the floor walls and ceding 



live, and the earth's surface 



'/H,i I 300. 



*"' It'll/. (( 396-300. 



V Combustion," Magnus Maclean and Makila 



f Olugow November Jo. 1889 ; " Eleclrifica- 



J'l.' Magnus Maclean and Makila Goto, Pkilf- 



with the quadrant electrometer ; and more recent observations 

 with electrific.ition by flame absolutely excluded, throw doubt 

 on the old conclusion, that both in town and country negative 

 eiecirificition is The prevailing condition of natural atmospheric 

 air in the lower regions of the atmosphere. 



§ 3. The electric ventilation found in the Old College, and 

 described in § 299 of " Electrostatics and Magnetism," accord- 



• F.-. 

 Ihe cart 

 •erved i:. 

 ing from i - 

 during a fe 

 M'Te fre'i'! 

 1.. •- 



? "Kl- 

 Goto, I';. 

 Hon of Ah 



To electric 

 machine 



M 40 so CO 7C 

 . ONt METRE 



Fic. I, 



eo 90 foo 



tofkical Mafozitu, August 1B90. 



NO. 1290, VOL. 50] 



ing to which air drawn through .1 chink, less than i-inch wide, 

 of a slightly open window or door, into a large room, showed 

 the electrification which it had on the other side of the chink, 

 whether that w.is the natural electrification of the open air, or 

 positive or negative eleclrification produced by aid of a spirit 

 lamp and electric machine in an adjoining room, has been tried 

 again in the New College with quite corresponding results. It 

 has also been extended to the drawing in of electrified air through 

 a tube to the enclosure represented in Fig. I of the present 

 paper ; with the result that the water- dropping test indicated in 

 the sketch, amply sufficed to show the electrification, and verify 

 that it was always the same as that of the air outside. When 

 the tube was filled with loosely packed cotton-wool the electri- 

 fication of the entering air was so nearly annulled as to be 

 in-ensible to the test. 



§ 4. The object proposed for the experiments described in the 

 present communication was to find if a small unchanged portion 

 of air could be electrified sufliciently to show its electrification 

 by ordinary tests, and could keep its electrification for any con- 

 siderable time ; and to test whether or not dust in the air is 

 essential to whatever of electrification might be observed in such 

 circumstances, or is much concerned in it. 



§ 5. The arrangement for the experiments is shown in the 

 diagram, Kig. i. A A is a large sheet-iron vat inverted on a 

 large wooden tray H 11, lined with lead. Hy filling the tt.ay with 

 water the air is confined in the vat. There are two holes in the 

 lop of the vat: one for the watcr-dropper c, and one for the 

 charging wire I). Both the water-dropper, and the charging 

 wire, ending with a pin-point as sharp as possible, are insulated 

 by solid parafiin, which is surrounded by a metal tube, .is shown 

 in half size in Kig. 2. To siart with they were supported by 

 pieces of vulcanite embedded in paraffin. But it w.-is found that 

 after the lapse of some days (possil)ly on account of ozone 

 generated by the incessant brush discharges), the insulation had 

 utterly failed in both of them. The vulcinite i)leccs were then 

 taken out, and solid paraffin, with the metal guard-tube round 

 it to screen it from electrically influencing the water-dropper, 

 was substituted. This has proved quite sttisfactory : the water- 

 dropper, with the flow of water stopped, holds a positive or a 

 negative charge for hours. 



S 6. A qu.adrant electrometer E (described in " Electrostatics 

 and Magnetism " §S 346-353) was set up on the top of the vat 

 near the water-dropper, ns shown in Kig. I. It was used with 

 lamp and semi-transparent scale to indicate the difTerencc of po- 

 tential between the watcr-droppcr and Ihe vat. The sensibility 



