M''Clkli,and and Powkr — Electrification by Friction. 49 



Linen was used as the rubber, and curves were plotted for a number of 

 metals all taking a negative charge. Fig. 5 refers to platinum, and the other 

 curves were quite similar. At the lowest pressures reached the rate of pro- 

 duction of charge was very small. As the pressure increased, the rate of 

 production rose quickly to a maximum value which in all cases occurred at a 

 pressure of about '08 mm. of mercury. The rate of production decreased as 

 the pressure was further increased and became more or less steady about 

 •5 mm. 



FIG. 5. 



Platinum(-)on Linen. 



Pressure in mms . of Mercury. 



The remarkable feature of all the curves was the regularity with which 

 the maximum occurred about the same pressure. 



Similar experiments were next made using silk as the rubber. Fig. 6 

 refers to nickel which is again charging negatively. The curve shows two 

 maxima, one at a pressure of '08 mm. and the other at "03 mm. We obtained 

 the same type of curve showing the same maxima for platinum. Using a 

 different sample of silk, the maximum at 08 mm. disappeared, and that at 

 •03 mm. remained. The obvious suggestion is that the first sample of silk 

 was not pure. 



We now endeavoured to plot similar curves when the metal was charging 

 positively. As mentioned frequently above, it was easy to get a well-polished 

 metal surface charging positively with the rubbers we used; but in these 

 experiments in air at low pressures it was only in the case of copper or iron 

 rubbed with linen that we were able to get a positive charge on the metal. 

 When these metals were charging positively, an increase in the rate of pro- 

 duction took place as the pressure was increased from very low values, hut 



