I 



ELECTRIFICATION OF AIR, VAPOUR OF WATER, AND OTHER GASES. 191 



15. With such a filter as this it is possible to determine the quality of the natural 

 electricity of the atmosphere, and it may be desirable that it should be used for that 

 purpose in meteorological observatories. 



GREATEST ELECTRIFICATION WHICH WE COULD COMMUNICATE TO A LARGE QUANTITY 



OF AlR BY ONE Oil MORE ELECTRIFIED NEEDLE POINTS ( 16-24). 



1G. The first apparatus used is shown in fig. 1. It consisted of a metal can, D, 

 48 centims. high and 21 centims. in diameter, supported by paraffin blocks, and con- 

 nected to one pair of quadrants of a quadrant electrometer. It had a hole at the toj 

 to admit the electrifying wire, which was 531 centims. long, hanging vertically within 

 a metallic guard-tube, B. This guard-tube was always metallically connected to the 

 other pair of quadrants of the electrometer, and to its sheath and to a metallic screen 

 surrounding it, which is not shown in the diagram. This prevented any external 

 influences from sensibly affecting the electrometer, such as the working of the electric 

 machine, A, which stood on a shelf five metres above it. 



17. The experiment is conducted as follows : One terminal of the electric machine 

 is connected with the guard-tube and the other with the electrifying wire, which is 

 tipped with needle points or tinsel, and which is let down to place the point or points 

 nearly in the centre of the can. The can is temporarily connected to the sheath of the 

 electrometer. The electric machine is then worked for some minutes, so as to electrify 

 the air in the can. As soon as the machine is stopped, the electrifying wire is lifted 

 clear out of the can. The can and the quadrants in metallic connection with it arc 

 disconnected from the sheath of the electrometer, and the electrified air is very rapidly 

 drawn away from the can by a blowpipe bellows, arranged to suck. This releases 

 the opposite kind of electricity from the inside of the 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. 



18. We tried different lengths of time of electrification and different numbers of 

 needles and tinsel, but we found that one needle and four minutes' electrification 

 gave as great electrification as we could get. The greatest deflection observed was 

 936 scale divisions on a half millim. scale put up in the usual way, with lamp at a 

 distance of about a metre from the electrometer. To find from this reading the 

 electric density of the air in the can, we took a metallic disc of 2 centims. radius, 

 attached to a long varnished glass rod and placed at a distance of 1'45 centims. 

 from another and larger metallic disc. This small air condenser was charged from 

 the electric light conductors in the laboratory to a difference of potential amounting 

 to 100 volts, or % of an electrostatic unit. 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 electrostatic capacity. 



