104 On the Generation of Statical Electricity. 
er will be supposed to change its position. A certain quantity of 
induced positive electricity being evolved on the second surface 
of the glass, by the action of the first rubber as already discuss- 
ed; this passes with the glass surface in its revolution, to the me- 
tallic face of the second rubber, where it necessarily becomes ab- 
sorbed by the negative electricity of that rubber, and the free 
positive electricity, generated by the first rubber on the first sur- 
face, is thus, to a great extent, rendered neutral. If the metallic 
faces of the two rubbers be in connection, as is the supposition, 
the glass surfaces will thus be brought back nearly to their primi- 
tive state, and the product of the action of the first rubber ceases 
almost entirely to exist. 
It follows from this, that it is important that the rubbers be 
placed opposite to each other ; a slight variation is not, however, 
very perceptible, by reason of the vibrating molecules of the 
glass, continuing to produce positive electricity, after passing to 
a certain distance the exciting points of the rubber. ‘That this 
is a correct hypothesis may be shown by the following experi- 
ment: let the back of the hand be held near the second surface 
of the glass plate of the machine; on rubbing the opposite sur- 
face with a piece of silk, every motion of the rubber, will be 
distinctly and instantly perceived; hence, if the electricity of 
the surfaces ceases to be generated, when the portions of glass 
pass beyond the exciting points of the rubbers, it follows, that if 
one of the rubbers be somewhat in advance of the other, the 
electricity induced by the remaining rubber, will be mostly ab- 
sorbed, and the practical action of this rubber destroyed, which 
for a slight variation is not the case. 
The length of the rubbers will now be determined. In cyl- 
inder machines, the rubbers should extend so as to rub the en- 
tire exposed surface of glass; the ordinary practice of limiting 
their length to a portion of the surface, appears to be deficient in 
principle. For, let it be supposed that such is the case; then 
those parts of the exposed glass surface, not subjected to the rub- 
bing action, hence not yielding electricity, must conduct off a 
small quantity of that produced elsewhere. But should the rub- 
bers extend to the axis, an additional quantity will be evolved, 
and if the axis abstract a portion, on account of its proximity, it 
will be but a small part of this additional quantity. It cannot 
be supposed, that in exciting those portions of the glass near the 
axis, the glass itself becomes a better conductor. 
