39^ Injtrumftilsfor exhiUting veal F.lcSJr'icitift. 



of elc£\ricity having been previoufly made. The former made fevcral valuable obferva- 

 tioiis and experiments to inveiHg-.itc the caufcs and remove the uncertainty produced by 

 this fpontaneous clc£lricity ; and the latter prefented to the Royal Society * an account of 

 a new inftniment called a collcftor of eleftricity, in which, as he aflerts, this imperfccliou 

 does not take place. It eonfifts of a plate of tin, fupported by two upright fticks of glafs ; 

 on each fide of which plate are two frames of wood, covered witli gilt paper, which do not 

 touch the tin plate, but ftand parallel to it at a fmall diftancc. Thefc frames are fallencd 

 to the platform of the inftrumeru by hinges; fo that if eleflricity be communicated to the 

 plate, it will receive a large quantity without any confiderable intenfity, becaufe its capacity 

 is much augmented by the vicinity of the plane of gilt paper on each fide. But if thefc 

 planes be thrown back into the horizontal pofition, which is eafily done by means of their 

 hinges, the eleflricity, which before was compenfated in the plate, will have its intenllty 

 greatly increafed. An elc£lrometcr connefled with this plate will therefore fhew figns of 

 cleflricity by means of a communication made between a large flock of ele£lricity, and the 

 tin plate in its firll pofition, though the intenfity of that flock may have been too fmall to 

 have a(Fe£led the elcflrometer without this contrivance. 



It does not appear, in the author's defcription of this indrumcnt, that it removes 

 the equivocal effedt of the doubler; for it is evident that it does not, in its fimple pro- 

 cefs, enter the province of the doubler in which this efFcifl takes place. The doubler 

 requires fix or feven turns before it will exhibit fpontaneous electricity ; at which period 

 the firft charge is magnified above twelve thoufand times : but his fimple inftrument will 

 fcarcely exceed one hundred times, and therefore requires the ele£lricity to be one hundred 

 and twenty times as (Irong as that which caufes the uncertainty of the doubler. Whence it 

 may be inferred, that the doubler would have aifled unequivocally with all fuch eledlricitics 

 as this inftrument is capable of exhibiting. 



But the fpinning inftrument, as Mr. Cavallo in the foregoing letter juftly obferves, does 

 nothing more than colle£l a confiderable quantity of dilFufed eledlricity into a fmall compafs, 

 though it does not appear to be exaftly the fame oftice as is performed by M. Volta's con- 

 denfer, or his colle£Vor, as defcribed by their refpeflive inventors. For thofe inftruments, 

 if very fmall, may exhibit very little intenfity in conjundtion with an elc£lrometer of con- 

 fiderable furface; whereas the fpinning inftrument, befides its facility in performing the 

 operations, may be confidered as pofTcQing a furface indefinitely great. Thefe advantages, 

 fuch as they are, conftitute all the difference I perceive between them. 



Mr. Cavallo's multiplier, defcribed in the third volume of his Complete Treatife on Elec- 

 tricity, which I had miftakenly compared with the fpinning inftrument, confifts of four me- 

 tallic plates. A, B, C, and D. The plate A is fupported on a ftick of glafs fixed in the 

 platform of the inftrument. The plate B is alfo fupported on a ftick of glafs which is fixed 

 on a radius or arm, by means of which it can be brought very near and parallel to A, and 

 again removed, fo as to touch the third plate C, by means of a wire projcfting from B. 

 The plate C is fupported, like A, by a ftick of glafs fixed in the platform. And, laftly, the 

 plate D is fixed by a metallic ftem upon a fiiding-piece, by means of which it can be brought 

 very near and- parallel to C, and occafionally removed. 



' Fhjtorophical Tranfaftions for 1788. 



The 



