102 On the Generation of Statical Electricity. 
the plate machines, will be examined. Theoretically, the num- 
ber of rubbers is unlimited ; for if one produces a certain effect, 
six would produce six times that effect, if the electricity be re- 
moved as rapidly as evolved ; but in practice, the number is neces- 
sarily limited, and this limit depends, collaterally, on the size of 
the plate or cylinder, and the convenience of construction. COryl- 
inder machines have but one rubber, which arrangement may 
have had its origin, in the larger machines, merely from the 
slightly increased difficulty of construction. This arrangement 
necessarily lessens their power one half. 
Plate machines have generally two pairs of rubbers, or four in 
all; in large plates, this number might be increased to three or 
four pairs, with corresponding increase of power; but the labor 
of working the machine would increase in the same ratio ; which, 
in this kind of machines, is a material circumstance. However, 
with rubbers constructed after a manner to be described, the labor 
caused by two rubbers, is but little greater than that caused by 
one, of the common construction. 
The action of double rubbers will now be discussed. A single 
rubber evolves a certain quantity of electricity; one surface of 
the plate being thus excited, the inductive influence causes a cer- 
tain amount of positive electricity to become free on the opposite 
face; which acts also by its induction on that of the opposite 
surface, and increases its amount; and this action and reaction 
between the surfaces, continue until an equilibrium is established ; 
the result being, that with the ordinary glass plates the original 
electricity generated is nearly doubled in its amount. If in this 
condition, the positive induced electricity of the second surface be 
removed, it will leave the corresponding quantity of negative elec- 
tricity on this surface of the glass, which will neutralize the oppo- 
site positive surface; and nearly all signs of excitation, on such 
surface, will consequently cease. By continuing this process, the 
second surface of the glass gradually ceases to give off electricity ; 
and the quantity generated on the first surface, not being increas- 
ed by induction, becomes comparatively feeble in itsaction. The 
plate has now become charged, in a manner similar to that ina 
Leyden jar; and if removed and placed on a ring of metal, a cor- 
responding ring being then placed upon it, and opposite to the 
first, by forming a connection between the two, a strong dis- 
charge will take place, and the plate resumes its first condition. 
