8 
Mr, Cavallo’s Obfervations on 
feparated from the femi- conducting plane, and being prefented 
to an electrometer will electrify it much more than if it had 
not been placed upon the above-mentioned plane. 
The principle on which the aCtion of this apparatus depends 
is, that the metal plate, whilft {landing contiguous to the 
iemi-conduCting plane, will both abforb and retain a much 
greater quantity of electricity than it can either abforb or 
retain when feparate, its capacity being increafed in the former, 
and diminilhed in the latter cafe. 
i 
Whoever confiders this apparatus, will eaiily find, that its 
office is not to manifeft a fmall quantity of electricity, but 
to condenfe an expanded quantity of eleCtricity into a fmall 
fpace ; hence, if by means of this apparatus one expeCted to 
render more manifeft than it generally is, when communicated 
immediately to an electrometer, the eleCtricity of a fmall tour- 
malin, or of a hair when rubbed, he would find himfelf 
miftaken. 
It is Mr. Bennet’s doubler that was intended to anfwer 
that end ; viz, to multiply, by repeated doubling, a fmall, and 
otherwife unperceivable, quantity of electricity, till it became 
fufficient to affeCt an electrometer, to give fparks, &c. The 
merit of this invention is certainly confiderable ; but the ufe of 
it is far from precife and certain. 
This apparatus confifts of three brafs plates, which we lhall 
call A, B, and C ; each of which is about three or four inches 
in diameter. The firft plate A is placed upon the gold-leaf 
electrometer, or it may be fupported horizontally by any other 
infulating ftand, and its upper part only is thinly varnilhed. 
The fecond plate B is varniflied on both fides, and is furnifhed 
with an infulating handle, which is fattened laterally to the 
edge of it. The third plate C is varnifhed on the under fide 
only. 
