﻿the Electrification of Air, 229 



difference of potential between the water-dropper and the vat. 

 The sensibility of the electrometer was 21 scale-divisions 

 (half-millimetres) per volt ; and as the scale was 90 centim. 

 long, difference of potentials up to 43 volts, positive or nega- 

 tive, could be read by adjusting the metallic zero to the 

 middle of the scale. A frictional plate-electric machine was 

 used, and by means of it, in connexion with the pin-point, 

 the air inside the vat could be electrified either positively or 

 negatively. 



§ 7. The vat was fixed in position in the Apparatus Room 

 of the Natural Philosophy Department of the University of 

 Glasgow on the 13th of December, 1893, and for more than 

 three months the air inside was left undisturbed except by 

 discharges from the pin-point through the electrifying wire, 

 and by the spray from the water-dropper. Thus the air was 

 becoming more and more freed of dust day by day. Yet at 

 the end of the four months we found that the air was as easily 

 electrified, either positively or negatively, as it was at the 

 beginning ; and that if we electrify it strongly by turning 

 the machine for half-an-hour, it retains a considerable portion 

 of this electrification for several hours. 



§ 8. Observations were taken almost daily since the 13th 

 December ; but the following, taken on the 8th of February, 

 the 12th of March, and the 23rd of April, will serve as 

 specimens, the results being shown in each case by a curve. 

 At all these dates the air must have been very free from dust. 

 Both during the charging and during the observations the 

 case of the electrometer and one pair of quadrants are kept 

 metallically connected to the vat. During the charging the 

 water-dropper and the other pair of quadrants were also kept 

 in connexion with the vat. Immediately after the charging 

 was stopped the charging wire was connected metallically to 

 the outside of the vat, and left so with its sharp point 

 unchanged in its position inside the vat during all the 

 observations. 



§ 9. Curve 1. February 8, 1894. — The friction-plate 

 machine was turned positive for half-an-hour. Ten minutes 

 after the machine stopped the water-dropper was filled and 

 joined to one pair of quadrants of the electrometer, while the 

 other pair was joined to the case of the instrument. The 

 first reading on the curve was taken four minutes after- 

 wards, that is fourteen minutes after the machine stopped 

 running (18 volts). 



Curve 2. March 3, 1894. — The friction-plate machine was 

 turned positive for five minutes. The water-dropper was 

 filled and joined to the electrometer immediately after the 



