﻿20 EQUIPMENT AND WORK OF AN AEEO-PHYSICAL OBSERVATORY. 



matter in water is enough to produce a marked change in the electrifica- 

 tion, and that therefore no study of atmospheric electricity will be com- 

 plete which does not take into account the degree of purity. Kelvin 

 from laboratory experiments (see paper read before British Association 

 at Oxford ; also " Nature," July 19, 1894) concludes that the air does not 

 retain a negative electrification so long as it retains a positive. Kelvin 

 says, " the equilibrium of electrified air within a space enclosed by a fixed 

 bounding surface of conducting material presents an interesting illustra- 

 tion of elementary hydrostatic principles. The condition to be fulfilled 

 is simply that 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 for want of knowledge ; but it is quite 

 possible that experiment will prove that it is not accurately true." " On 

 the supposition of electric density uniform throughout the spherical en- 

 closure, each cubic centimetre of air experiences an electrostatic force 

 toward the boundary in simple proportion to the distance from the cen- 

 tre and amounting at the boundary to nearly 10 per cent, of the force of 

 gravity upon it ; ..." " Under natural conditions with great den- 

 sity there must be an important ponderomotive force quite comparable in 

 magnitude with that due to difference of temperature." . . . "Nega- 

 tively electrified air over negatively electrified ground with non-electrified 

 air above it in an absolute calm would be in unstable equilibrium." 



Lord Kelvin gives an estimate of the density and force in a given 

 enclosure. 



Let F equal the potential indicated by the water-dropper ; a equals 

 the radius of the spherical hollow (in which the air was) ; p equals elec- 

 trical density of air at distance r from center ; then from — 



— X'(f-T)* 



if p is constant, 



V= % r.pa^ 



P = 3V/2 T.ce. 



Suppose F equals 38 volts or 0.127 electrostatic units C. G. S.; a equals 

 50 cm. and /> = 2.4XlO~^ the electrostatic force at a distance 7^ being 



Hence a small body electrified with a quantity of electricity equal to 

 that possessed by a cubic centimetre of air and placed midway (r = 25) 

 between the surface and centre of the inclosure experiences a force equal 



