90 



Lord Kelvin and Mr. M. Maclean. [May 31 t 



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 afterwards 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 posi- 



