68 Observations on some Experiments in Electricity. 



or other revolving machine were substituted for the plate and its 

 movement were sufficiently rapid, we might conceive that the air put 

 in motion by friction, along its side, would produce revolution in any 

 light system of bodies suspended near its vertical face. But then 

 we should expect to find the revolution of the system coinciding in 

 direction with that of the wheel, as if the wheel and revolving arms 

 were connected by bevel gearing. 



In the experiment above described the revolution is always in the 

 opposite direction ; that is, the ball nearest to the plate moves in 

 such a manner as to meet the motion of the plate itself. If in- 

 stead of two arras and balls we use four of each, at right angles, the 

 result is the same, but the effect more immediate ; the revolution 

 takes place without the oscillations above mentioned. I have said 

 that the supporting stand was insulated. But, the experiment suc- 

 ceeds equally well when the cap containing the point on which the 

 needle rests is connected with the groimd. 



The explanation of this experiment appears to depend on the dif- 

 ferent degrees of intensity with which the plate acts upon the air at 

 different distances from the rubber. As the glass in its revolution 

 becomes gradually divested of its charge, the part opposite to a light 

 body near the rubber, will be capable of furnishing to the air a great- 

 er portion th^n one more remote 5 and as air, when dry, appears to 

 conduct electricity much in the same manner as liquids conduct heat, 

 that is by means of its mobility — the currents caused near the ma-r 

 chine will be more rapid where the accumulation is greatest, hence 

 those balls and arms nearest to the rubber are most vigorously re- 

 pelled. The currents become perceptible by the aid of a lighted 

 taper held near the plate. 



10. .Amalgam, . 



In some experiments on coatings for the rubber, I have found 

 plumbago reduced to an impalpable powder, to answer the purpose 

 of an amalgam, nearly or quite as well as the compounds of zinc, 

 tin, and mercury recommended by electricians. The plumbago 

 should be free fromsilicious matter or other impurities which may 

 SiCralcJi the glass. 



Note. Throughout these observations the itims positive and neg- 

 ative have b.een employed, in conformity with common usage, rathei' 

 tt?,an with the conviction of their strict propriety. 



