﻿232 Lord Kelvin and Mr. Magnus Maclean on 



Curve 4. April 23, 1894. — The friction- plate machine 

 was turned positive for 30 seconds, with water-dropper 

 running and joined to the electrometer. 20 seconds after the 

 machine stopped the spot appeared on the scale, and the 

 reading 1^ minutes after the machine stopped turning is the 

 first point on the curve (7*3 volts). 



Curve 5. April 23, 1894. — The friction-plate machine 

 was turned negative for 30 seconds, with the water-dropper 

 running and joined to the electrometer. 10 seconds after- 

 wards the spot appeared on the scale, and the reading 70 

 seconds after the machine stopped turning is the first point 

 on the curve (7*6 volts). 



The curves show, what we always found, that the air does 

 not retain a negative electrification so long as it retains a 

 positive. We also found, by giving equal numbers of turns 

 to the machine that the immediately resulting difference of 

 potential between the water-dropper and the vat was greater 

 for the negative than for the positive electrification ; though 

 the quantity received from the machine was probably less in 

 the case of the negative electrification, because the negative 

 conductor was less well insulated than the positive. 



§ 10. On the 21st of March, two U-tubes were put in below 

 the edge of the vat, one on either side, so that it might be 

 possible to blow dusty, or smoky, or dustless air into the vat. 

 To one tube was fitted a blowpipe-bellows, and by placing it 

 on the top of a box in which brown paper and rosin were 

 burning, the vat was filled with smoky air. Again, several 

 layers of cotton-wool were placed on the mouth of the bellows, 

 so as to get dustless air into the vat. The bellows were worked 

 for several hours on four successive days, and we found no 

 appreciable difference (1) in the ease with which the air 

 could be electrified by discharges from the wire connected to 

 the electric machine, and (2) in the length of time the air 

 retains its electrification. 



But it was found that, as had been observed four years ago 

 with the same apparatus*, with the water-dropper insulated 

 and connected to the electrometer, and no electrification of 

 any kind to begin with, a negative electrification amounting 

 to four, five, or six volts gradually supervened if the water- 

 dropper was kept running for 60 or 70 minutes, through air 

 which was dusty, or natural, to begin with. It was also found, 

 as in the observations of four years ago, that no electrification 

 of this kind was produced by the dropping of the water 

 through air purified of dust. 



The circular bend of the tube of the water-dropper shown 

 * Maclean and Goto, ( Philosophical Magazine,' August 1890. 



