On the Generation of Statical Electricity. 107 
ed quantity of electricity, is inexpedient, as the additional amount 
thus evolved, does not compare with the increase of friction ; 
between zero of pressure and this limit, however, the quantity 
generated is directly proportional to the friction or pressure. 
Large plate machines, as will be seen hereafter, admit of less 
pressure on equal surfaces than cylinders, and consequently are 
less effective, as the limit above alluded to is not attained. 
Puares anp Cytinpers.—It is now proposed to examine into 
the comparative qualities of plates and cylinders, in order to de- 
termine their relative values as electrical generators. 
If an unpolished glass surface be employed, with an amalgam 
rubber, the electricity generated on the glass will be negative; if 
it be partly rough and partly polished, the rough parts will give 
out negative, and the polished positive electricity ; if equal in 
extent, they will neutralize each other’s action, and no effective 
result will be obtained. As the polished surface increases in ex- 
tent, the rough surface decreasing in the same ratio, the positive 
electricity acquiring the ascendancy, will increase in quantity 
until the glass becomes entirely polished, when its intensity will 
be ata maximum. It follows from this, that the fineness of the 
polish is a necessary qualification. Inequalities in the surface 
lessen the effect, by reason of the depressions not being acted 
upon properly by the rubber ; hence the surface should be ground 
smooth before polishing. The glass should be kept perfectly 
clean; as any substance adhering to its surface would cause the 
rubber to act on that in place of the glass, and by hypothesis, a 
diminished result would be obtained. Should such substance be 
taken from the rubber, it carries with it a portion of its negative 
electricity, thus neutralizing a portion of the positive surface.* 
F‘rom what precedes it appears, that plates have the advantage 
of admitting a finer polish and more uniform surface than cylin- 
ders; which latter, however, are usually thinner, and therefore 
the inductive action is stronger. It will be supposed that the 
advantages of each are in these respects balanced ; it remains to 
compare their powers. For this purpose, a plate of thirty six 
* Que lorsque l’un des corps soumis a l’expérience est entamé par l'autre, celui- 
ci, outre, l’électricité qui lui est propre, prend encore, avec la petite couche mince 
de la substance qu'il enleve, une portion d’électricité propre a cette derniére; de 
sorte que la sienne se trouvant modifieé, peutétre positive, nulle ou négative.—M. 
Becquerel, Vol. I, p. 122. 
