2S2 



NA TURE 



[July 19, 1894 



the equal and opposite quantity on the 

 inner boundary of the enclosing metal ; 

 and we therefore have the formula : — 



where V denotes the potential indicated by 

 the water-dropper, a the radius of the 

 spherical hollow, and f the electric density 

 of the air at distance r from the centre. 

 Supposing now, for example, p to be con- 

 stant from the surface to tlie centre (which 

 may be nearly the case after long electri- 

 tication as performed in our experiments), 

 we find V = ;iirpa'-' ; whence p = 3V/2Ta'-'. 



To particularise further, suppose the 

 potential to have been 38 volts or 0"I27 

 electrostatic c.g.s. (which is less than the 

 greatest found in our experiments) and 

 take <i = 50 cm. : we find p = 2"4. 10"^. 

 The electrostatic force at distance r from 

 the centre, being 'irpr, is therefore equal 

 to lo"* r. 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 experi- 

 ences a force equal to 2'4.lo~'.25, or 

 6 X lo"*, or approximately 6. 10"' grammes 

 weight. This is 48 per cent, of the force 

 of gravity on a cubic centimetre of air of 

 density i/Soo. 



§ 14. Hence we see that, on the suppo- 

 sition of electric density uniform through- 

 out the spherical enclosure, each cubic 

 centimetre of air experiences an electro- 

 static force towards the boundary in simple 

 proportion to distance from the centre, and 

 amounting at the boundary to nearly 10 



kept running for 60 or 70 minutes, through air which was dusty, 

 or natural, to begin with. It was also found, as in the observa- 

 tions of four years ago, that no electrification of this kind was 

 produced by the dropping of the water through air purified of 

 dust. 



The circular bend of the tube of the water-dropper shown in 

 the drawing was made for the purpose of acting as a trap to 

 prevent the natural dusty air of the locality from entering the 

 vat when the water-dropper ran empty. 



§ II. The equilibrum of electrified air within a space enclosed 

 by a fixed bounding surface of conducting material presents an 

 interesting illustration of elementary hydrostatic principles. 

 The condition to be fulfilled is simply that the surfaces of equal 

 electric "volume-density" arc surfaces of equal potential, il we 

 assume that the material density of the air at given temperature 

 and pressure is not altered by electrification. This .assump- 

 tion we temporarily make from want of knowledge ; but 

 it is quite possible that experiment may prove that it is 

 not accurately true ; and it is to be hoped that experimental 

 investigation will be made for answering this very interesting 

 question. 



§ 12. For stable equilibtium it is further necessary that the 

 electric density, if not uniform throughout, diminishes from the 

 bounding surface inwards. Hence, if there is a portion of non- 

 cleclrified air in the enclosure it must he wholly surrounded by 

 electrified air. 



§ 13. \Vc may form some idea of the absolute value of the 

 electric dcn.iiiy, and of the electrostatic force in different parts 

 of the enclosure, in the electrifications found in our experiments, 

 by considering instead of our vat a spherical enclosure of dia- 

 meter intermediate between the diameter and depth of the vat 

 which we used. Consider, for example, a spherical space 

 enclosed in metal of 100 cm. diameter, and let the nozzle of the 

 water-dropper be so placed that the stream breaks into drops at 

 Ihe centre of the space. The potential shown by the electro- 

 meter connected with il, being the difference between Ihe 

 polenliaU of the air at the boundary and at the centre, will be 

 Ihe difference of the potentials at the centre due respectively to 

 the total quantity of electricity distributed through the air and 



NO. I 290, VOL. 50J 



