Err,»»,^s Comparifi,, of the Ehrlrknl Muluplm- and Spimimg I„Jl,umi„t. 395 



fnends, at his houfe, nearly at the fame time, and in the fame year transferred to tlie cele- 

 brated Mr. Van Marum of Haerlcm, who now poflefTes it. From various other avocations 

 I was prevented from caufing any others to be made. It is not therefore wonderful that 

 the fame thought fliould fince have occurred to fo great a mafter of the fubjea as Mr. Ca- 

 vallo, who, in the third volume of his Ek-aricity, publiihed in 1795, gives a defcription and 

 ■engravings of an inftrument very different in form, but the fame in principle, &c." 



Now the error I allude to conCfts in the affeition that the tico i„jl,-u,nc,:U are the fame in 

 principle; and I flatter myfelf that the following explanation will be fufficient to (hew that 

 they are fo far from it as to be totally different in principle and in effed. 



Your inftrument does nothing more than colleft a confiderable quantity of diffufed elec- 

 tricity into a fmall compafs, which Is exatlly the fame office as is performed by Mr. Volta's 

 condenfer, or by my collcaor of ekaricity, which is defcribed in the PhilofophicalTranf- 

 aa.ons for the year ,788 ; whereas my multiplier renders an abfolutely fmall quantitv of 

 elear.c.ty man.feft, by accumulating a confiderable quantity of the contrary elearic'ity 

 Or, to be more explicit, your inftrument cannot communicate to the ekarometer a greater 

 quantity of elearlcity, or rather not nearly fo much of it, as is contained in the ekarified 

 body which IS to be examined ; whereas my multiplier accumulates a quantity of elearicitv 

 many umes greater than that of the body in queftion, and of courfe much more perceptible 

 Suppofe, for inftance, that a certain body, having its furface equal to that of the elearo- 

 meter which belongs to your inftrument, contain 100 parts of elearicityi and fuppofe. 

 likewife, that the faid ekarometer requires 200 fuch parts of ekaricity in order to (hew 

 any divergency. Now your inftrument will be found incapabk of manifefting the elec- 

 tricity of the given body; for, when the upper tinfoil fegments have conveyed the greateft 

 portion, or we may even fay all the 100 parts of ekaricity from the given body to the hook 

 with the ekarometer. they cannot, in the fubfequent rotations of the inftrument colled 

 any more. The ekarometer, therefore, being at moft loaded with 100 parts of ekaricitv 

 cannot (hew any divergency; fince, according to the fuppofition, at kaft 200 parts of 

 ekaricity are required in order to produce any divergency in it. 



But if the above-mentioned ekarified body be applkd to the plate A of my multiplier 

 and the inftrument be worked, the plate C, and of courfe an ekarometer which is con* 

 ..caed with It, will foon acquire 300 or 400 parts, or, in (hort, a much greater quantity 

 of eleariaty than is fuff.cknt to caufe a divergency in the ekarometer; becaufe, bj 

 working the inftrument, the original quantity of ekaricity in the plate A is not diminiftied 

 or removed from it ; but the contrary ekaricity (which is repeatedly and unlimitedly in- 

 duccd upon the plate 13 by the plate A) is accumulated upon the plate C, &c. 



„- „ „ ,, ■^■'■, I remain your obedient humble Servant, 



Wells-Ztreet, Oa. 30, ,797. ,,,_ CAVALLO. 



AFTER returning my fincerc thanks to Mr. Cavallo for the deteaion of the error he 

 mentions, .mo which I certainly (liould not have fallen if I had confulted his Treatifc it the 

 time the paper he alludes to was written, I fliall tak^ the prefent opportunity of makin. 

 fome remarks on the .nftruments we at prefent poffefs for manifefting the prefence and kind 

 cf weak clcarictics. Thcfe are : the condenfer, the fimpic doublcr, the doubler with me- 



3 ^ ^ chanifm, 



