35 '> Aaior. cf the Ek^fophort. 



placed upon it, and touclied by the finger, n portion of the elc£lricity may really pafs to the 

 metal at the few places of adual contact 5 and another part, if the intcnfity be llrong, may 

 Urikc through the thin plate of air jnterfperfed between the metal and the refinous face, 

 AVhat remains on the refinous furface wil! lofc by far the greateft part of its intcnfity, in 

 conlequcnce of its becoming a charge to that fmall plate of air, while the metallic plate, 

 poflefiing the oppofite part of the charpfc, will have its intcnfity equally low. During thcfe 

 charges, the fpark which padls from the metal mufl therefore be of the fame chara£ler as 

 that of the refinous plate ; in the fame manner as the outer coating of the jar emits pofitive 

 fparks during its tranfilion to the negative ftate. The commencing intcnfity of this fpark 

 cannot exceed that which belonged to the charge between the refinous furface and the lower 

 coating : but as the intcnfity requifitc to maintain a charge on the thin plate of air is ex- 

 tremely minute, by far the greater part of the eleftricity will be employed in conftituting 

 that charge, and the fpark given olT by the plate will be much {horter and more gradual than 

 if the fame quantity had pafl'ed at once from the plate in a flate of fimple eleflrization ; 

 that is to fay, the fpark will be apparently very fmall. But when the plate is raifcd up by 

 its infulating handle, part of the charge on the refinous furface will begin to acquire its former 

 llate with regard to the lower coating; and that which continues to be compenfated by the 

 upper moveable plate, will require a greater portion of uncompenfated eledlricity to main- 

 tain it. The intenfity of the plate will rapidly increafe as it rifes. It will throw out 

 fparks and ramifications to the lower plate, and to the furrounding bodies; and at a certain 

 dillance the whole remaining portion of its former charge may be tonfidered as fimple 

 elc£lricity, which will ilrike the finger at a greater dillance, and with more fuddennefs and 

 brilliancy. This fpark, though in facfl ccnfifting of lefs eledlricity than the former, will 

 neverthelefs be more perceptible both to the eye and the ear, and eonfequently will be 

 thought larger : the firft is the gradual explofion of a charge ; the fecond confifts of the 

 fudden efeape of a portion of fimple eleftricity. 



If we fuppofe the non-condufling matter of the eleclrophore to be extremely thin, and the 

 fpark when in contaft to be made to pafs from the upper to the lower metallic plate, the 

 eUcd. will be nearly the fame as the tranfition of the fliock from fuch a coated eleftric with 

 a charge of the quantity of eleftricity which pafTes ; and when the moveable plate is raifed, 

 the efie£l will be fimilar to that of a fpavk from the prime conductor. Or rather, per- 

 haps, by a comparifon familiar to ele£lricians, the firft may be confidered as the (hock 

 from a Large battery charged bj only one turn of the handle of the machine, and the latter 

 as the fhock from a fmall jar capable of being charged high by the very fame quantity of 

 ele£lric matter. It is certain that the fliock, as well as the fpark from the battery, would in 

 thefe circumftances be inconfiderable, though the efl"e£ls of the jar might be very ftriking. 



When the eledtrophore is thus compared with the jar, its charge will be found to be won- 

 derfully fmall, as it confifts of no more eledricity than, when uncompenfated, would pafs 

 off iit the fimple fpark which was obtained. 



I found that two fquare inches of Mufcovy talc, about one-hundredth part of an inch 

 thick, which is an exceedingly good non-condu£lor, required, when coated with tinfoil, 

 one turn of a fmall cylinder to difcharge through one-tenth of an inch ; and one turn of 

 the fame cylinder charged a fimple conduftor of about fix fquare feet furface. To as to 

 give a fpark about nine inches long. Now if we alTume the quantities of eledricity in 

 4 condudlora 



