TRANSACTIONS OF SECTION A. 631 



then NTorked for some minutes, so as to electrify the air in the can. As soon as 

 the machine is stopped the electrityin.^ wire is lifted clear out of the can. The 

 can and the qnadrants in metallic connection with it are disconnected from the 

 case of the electrometer, and the electrified air is very rapidly drawn away from 

 the can by a blowpipe bellows arranged to exhaust. This releases the opposite kind 

 of electricity from the inside of tlie can, and allows it to place itself in equilibrium 

 on the outside of the can and on the insulated quadrants of the electrometer in 

 metallic connection with it. 



§ 3. We tried diftercnt lengths of time of electrification, and different numbers 

 of needles and tinsel, but we found that one needle and four minutes of electrifica- 

 tion gave nearly maximum effect. The greatest deflection observed was 936 scale 

 divisions. To find, from this reading, the electric density of the air in the can, we 

 took a metallic disc, of 2 cm. radius, attached to a long varnished glass rod, and 

 placed it at a distance of 1'45 cm. from another and larger metallic disc. This 

 small air condenser was charged from the electric light conductors in the labora- 

 tory to a difference of potential amounting to 100 volts. The insulated disc thus 

 charged was removed and laid upon the roof of the large insulated can. This 

 addition to the metal in connection with it does not sensibly influence its electro- 

 static capacity. The deflection observed was 122 scale divisions. The capacity 



of the condenser is approximately ^!~-- - The quantity of electricity 



with which it was charged was - — x ^- = -—- electrostatic unit. Hence the 



1'45 dOO 4'35 



quantity to give 936 scale divisions was x =1'7637. 



^ ^ ^ 4-35 122 



The bellows was worked vigorously for two and a half minutes, and in that 



time all the electrified air would be exhausted. The capacity of the can was 



16,632 cubic centimetres, which gives, for the quantity of electricity per cubic 



centimetre, ,„,,^,,- = 1-06 x 10^''. The electrification of the air in this case was 

 lobd2 



positive : it was about as great as the greatest we got, whether positive or 



negative, in common air when we electrified it by discharge from needle points. 



This is about four times the electric density which we roughly estimated as about 



the greatest given to the air in the inside of a large metal vat, electrified by a 



needle point and then left to itself, and tested by the potential of a water-dropper 



with its nozzle in the centre of th'5 vat, in experiments made two years ago, and 



described in a communication to the Royal Society of date May 1894.1 



§ 4. In subsequent experiments electrifying common air in a large gas-liolder 

 over water by an insulated gas flame burning within it with a wire in the interior 

 of the flame kept electrified by an electric machine to about 6,000 volts, whether 

 positively or negatively, we found as much as To x 10~' for the electric density 

 of the air. Electrifying carbonic acid in the same gas-holder, ivhetker positively or 

 negatively, by needle points, we obtained an electric density of 22 x 10~'. 



§ 5. AVe found about the same electric density (2-2 x 10~') of negative 

 electricity in carbonic acid gas drawn from an iron cylinder lying horizontally, and 

 .allowed to pass by a U-tube into the gas-holder without bubbling througli the 

 water. This electrification was due probably not to carbonic acid gas rushing 

 through the stopcock of the cylinder, but to bubbling from the liquid carbonic acid 

 in its interior, or to the formation of carbonic acid snow in the passages and its 

 subsequent evaporation. When carbonic acid gas was drawn slowly from the 

 liquid carbonic acid in the iron cylinder placed upright, and allowed to pass, 

 without bubbling, through the (J -tube into the gas-holder over water, no electrifi- 

 cation was found in the gas unless electricity was communicated to it from needle 

 points. 



§ 6. The electrifications of air and carbonic acid described in Sections 4 and 5 

 were tested, and their electric densities measured, by drawing by an air pump 



' On the Electrifcation of Air. By Lord Kelvin and Magnus Maclean. 



