92 Lord Kelvin and Mr. M. Maclean. [May 31, 



But it was found that, as had been observed four years ago -with 

 the same apparatus,* with the water-dropper insulated and connected 

 to the electrometer, and no electrification of any kind to begin with, 

 a negative electrification amounting to four, five, or six volts gradu- 

 ally supervened if the water- dropper was kept running for 60 or 70 

 minutes, through air which was dusty, or natural, to begin with. It 

 was also found, as in the observations 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. 



11. The equilibrium of electrified air within a space enclosed by a 

 fixed bounding surface of conducting material presents an interesting 

 illustration of elementally hydrostatic principles. The condition to 

 be fulfilled is simply that the surfaces of equal electric " volume- 

 density " are surfaces of equal potential, if we assume that the 

 material density of the air at given temperature and pressure is not 

 altered by electrification. This assumption 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 equilibrium 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- 

 electrified air in the enclosure it must be wholly surrounded by 

 electrified air. 



13. We may form some idea of the absolute value of the electric 

 density, 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 diameter 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 the centre of the space. The potential shown by the electro- 

 meter connected with ifc, being the difference between the potentials 

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

 the potentials at the centre due respectively to the total quantity of 

 electricity distributed through the air and the equal and opposite 

 quantity on the inner boundary of the enclosing metal ; and we there- 

 fore have the formula : 



( a lr"- r z 

 = 4?r p( 



Jo \ r a 



* Maclean and Goto, ' Philosophical Magazine,' August, 1890. 



