M'Clkli.anp and Powbe — Electrification by Friction. 



A:\ 



where the charge on the metal was negative, for the variation of production 

 with speed is different in the two cases. As an example of tin- results when 

 the metal is positive we give curves (rig '1) foi copper rubbing <>ii silk, the 

 different curves corresponding to different pleasures between t he copper and 

 the silk. Somewhat similar curves were plotted for zinc and iron, the charge 

 on the metal being positive in both eases. 



'20 



wo 



F I G. 2. 



Copper (+) on Silk. 



Pressure 



120 grams 



2S2 28 



As the speed of rubbing increases, the production increases to a certain 

 point; a maximum rate of production is reached which is not altered by a 

 further large increase of the speed. The maximum rate of production of a 

 positive charge on copper was reached at a speed of about '20 feet per second. 

 and no further increase of production took place when the speed was altered 

 to 56 feet per second. The speed required to give the maximum rate of pro- 

 duction of positive charge on zinc or iron was greater than in the case of 

 copper, being about 30 feet per second. 



The etl'ects of different pressures between the metal and the silk are also 

 shown by the curves on tig. 2, the production of electricity increasing with an 

 increase of pressure whatever may be the speed of the rubbing surface. The 

 maximum rates of production at different pressures are roughly proportional 

 to the pressures. 



In similar experiments with metal caps which gol a negative charge, the 

 results were less certain. Sometimes numbers would be measured which 

 showed that the rate of production tended towards a maximum at high 

 speed, and in other cases the rate of production increased even more rapidly 

 than the speed. 



