﻿EQUIPMENT AND WORK OF AN AERO-PHYSICAL OBSERVATORY. 21 



to 2.4X10~^X25 or 6 X 10~^, or approximately 6.10~^ grammes, which is 

 4.8 per cent, of the force of gravit}^ on a cubic centimetre of air of density 

 1/800. " During a thunder storm," says Kelvin, " the electrification of air, 

 or of air and the watery spherules constituting cloud, need not be enor- 

 mously stronger than that found in our experiments." (In the experiments 

 referred to below higher values were obtained.) " This we see by con- 

 sidering that if a uniformly electrified globe of a metre diameter produces 

 a difference of potential of 38 volts between its surface and centre, a globe 

 of a kilometre diameter, electrified to the same electric density, reckoned 

 according to the total electricity in any small volume (electricity of air 

 and of spherules of water if there are any in it), would produce a differ- 

 ence of potential of 38,000,000 volts between its surface and centre. In 

 a thunder-storm, flashes of lightning show us differences of potentials of 

 millions of volts, but not perhaps of many times 38,000,000 volts, between 

 places in the atmosphere distant from one another by half a kilometre." 

 One may go farther and say that in this electrification of air may lie 

 the possibility of principles valuable in aerial navigation, for as in the 

 ordinary Thomson electrometer the aluminum needle between the quad- 

 rants moves always from the region of high (positive) to the region of 

 low (negative) potential, so if the potentials are sufficiently high a charged 

 body free to move in the air will move from the place of high to the place 



/OO^OO t/^LTS 



•c 



/OOOOO \/OLTS 



> 



1 . / / / / / 



of low potential. Thus in the accompanying diagram if A represents a 

 highly charged (and by this we mean voltages in the hundred thousands) 

 insulated conductor, B a mobile conductor charged equally high and 

 same sign as A^ then C an oppositely charged mass, B will move from 

 ^ to C. 



