Manifestation of small Portions of the Electric Fluid. 49 
plate B of same diameter serving as the carrier; and the com- 
bined condensers cC (the former one inch and three quarters, 
the latter six inches in diameter) as the reservoir. 
“In using this instrument, it is obvious that the subject in- 
tended for examination must be breaght in contact with the 
source A, where the fluid thus communicated will undergo, if 
the air be dry and the insulation good, but very little dissipation 
for a minute and upwards. Every time the carrier B_ passes by 
(that is, directly opposite and within a suitable distance of) this 
Source, the projecting pin of B the carrier must touch the little 
lever of the perpendicular uninsulated brass rod D, and part it 
instantaneously: thus, without depriving the source of any por- 
tion of its fluid, there is imparted to the carrier an apparently 
equa} portion of the opposite electricity, which it deposits on the 
combined reservoir cC by lifting the projecting lever of the 
smaller plate c. 
“¢ | have made repeated experiments with this instrument, and 
found that ifthe reservoir C be full six inches diameter and pro- 
perly adjusted, the combined reservoir ¢C will retain, provided 
the communicated electricity be sufficiently weak, about 250 
deposits: but here it should be observed, that if the surface 
of the body presented to the sowrce for examination, be great, 
and its electricity weak, thé carrier should be set directly oppo- 
site the source, while the contact is made ;—the source and car- 
rier forming thus, in conjunction, a small condenser.” 
«December 13, 1810. 
“In reply to your Lordship’s observations, I can assure you 
that I have left nothing undone to bring this instrument to all 
possible perfection. 1 found by repeated trials that about 250 
deposits were the absolute me plus ultra. When more were at- 
tempted, so as to attain the complete maximum of the reservoir, 
the instrument appeared less perfect, the result being sometimes 
equivocal ; for do what we please, the electric substances which 
must necessarily be used to insulate the plates, will on certain 
occasions retain for a considerable time a sufficient residuum to 
affect our electrometer: how, therefore, should the communi- 
cated electricity be distinguished from the zzherent ? Upon no 
occasion, then, should the operator proceed without previously 
ascertaining what number of revolutions may, at that time, be 
commanded, I recollect, one frosty day, to have produced spon- 
taneous electricity (whether from the air, the earth, or pillars, I 
cannot teil,) by 230 deposits, which consequently obliged me to 
Jimit my revolutions to 200, during an experiment which I was 
then performing. 
* From all the foregoing facts it would appear, my Lord, that 
a larger reservoir than that of C would be quite unnecessary, the 
Vol, 52, No,243, July 1818. D com- 
