ELECTRIFICATION OF AIB, VAPOUR OF WATER, AND OTHER GASES. 193 

 The quantity of electricity with which it was charged was therefore 



T. X ^ = v~ C.G.S. electrostatic unit. 



Hence, the quantity on the can and connected metal to give 936 scale divisions was 



1 936 

 4-3" * 129 = 1*7637 C.G.S. electrostatic units. 



The capacity of the can was 16,632 cub. centims., which gives, for the quantity of 

 electricity per cub. centim. 



= 1-06 X 10- 4 of the C.G.S. electrostatic unit. 



16632 



19. 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 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 the vat, in experiments made more than three years ago and 

 described in a communication to the Royal Society of date May, 1894.*" 



20. To enable us to remove the electrified air quickly from the can, the following 

 modification was adopted : The can was suspended vertically by three stout silk 

 threads (S, S, S, fig. 2) which had been previously soaked in melted paraffin ; it was 

 quite open at the bottom but closed at the top, with the exception of a central 

 aperture for the piston-rod of a piston, P. The piston was of wood encased in lead, 

 and was free to move up and down in the can by the movement of the paraffined 

 silk cord, C, over the pulleys, F, F. The can and the piston and piston-rod were 

 connected metallically by spiral springs of fine wire. The can was surrounded by a 

 metallic guard-screen, G, kept in connection with the sheath of the quadrant electro- 

 meter, and with the sheath of a vertical electrostatic voltmeter, and with one 

 terminal, B, of an electric machine. The other terminal of the machine was con- 

 nected to an insulated needle-point inside the can and to the insulated terminal of 

 the voltmeter. 



21. By working the machine the needle was kept charged positively or negatively 

 at 12,000 volts for four minutes. The air inside the can became charged similarly 

 by the brush discharge from the needle point. As soon as possible after stopping 

 the machine, the needle was removed and A and B were joined. The wire, W, from 

 the can was disconnected from the guard screen, G, and then attached to the 

 electrometer terminal, O, after which this terminal was insulated and the downward 



* "On the Electrification of Air," by Lord KELVIN and MAGNUS MACLEAN. 

 VOL. CXCI. A. 2 C 



