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. 
n 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 arms 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 ground. 
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 than one more remote; 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- 
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 from silicious matter or other impurities which may 
scratch the glass. 
Norte. Throughout these observations the terms positive and neg- 
atwe have been employed, in conformity with common usage, rather 
than with the conviction of their strict propriety. 
