New EleBrka! Injlrument. 17 



e!ei£lrlcity fliould be totally removed, he would have recourfe to a fimple multiplier, and 

 not a doubler. I did not apprehend the contrivance from this flight defcription, which at 

 my requeft, however, he readily extended, and convinced me of tlie full value of his in- 

 vention. I then mentioned my intention to conftru£l; an inftrument on that principle, 

 wluch foon afterwards I did. It was (hewn to Sir Jofeph Banks and his friends, at his 

 houfe, nearly at the fame time, and in the fame year transferred to the celebrated Mr. van 

 Marum of Harlem, who now poflefles 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 mailer of the fubjeft as M. Cavallo, who in the 

 third volume of his Eleftricity, publifted in 1795, gives a defcription and engravings of an 

 inftrument very different in form, but the fame in principle. The form I have given it, 

 which is all the fliare I had in the contrivance, is more calculated fos a fpeedy repetition 

 of the'procefs than M. Cavallo's : but his inftrument is much eafier to be made by one who 

 is not profeffionally a workman. Mr. Bennet's notion with regard to form was very differ- 

 ent from either. Both inftruments have the property, that a fimple or fmall aggregate of 

 elearicity will not be multiplied unlefs its Intenfity exceed that of the adherent eiedricity 

 of the plates, fuppofed to be contrary ; and both are capable of deftroying their own elec- 

 tricity, and exhibiting unequivocally what they receive by communication, however weak it 

 may naturally be, provided the fupply be kept up. They are both alfo limited as to the 

 extent of their multiplication ; that is to fay, the eft"e£l will be denoted by a fixed number 

 or multiplier, fo long as the neareft diftance of the plates continues unaltered. For ex- 

 ample, if this number were 100, the inftrument would (hew no eledlricity which was lefs 

 intenfe than the -tist^h. part of what the ele£trometer demanded to ad upon it. This co- 

 efficient is ncvertkelefs capable of being enlarged at pleafure, by adjufting the plates to a 

 lefs diftance afunder. 



Fig. 8. reprefents a vertical fedlion of the inftruments. A is a metallic vafe, having a 

 long fteel axis which pafles through an hole in the ftand H at K, and refts on its pointed end 

 in an adjuftable focket at C. The ufe of the vafe is, by its weight, to preferve, for a confi- 

 derable time, the motion of fpinning which is given by the finger and thumb applied to the 

 nob at the top of the inftrument. The (haded parts D and E reprefenc two circular plates of 

 glafs nearly i^ inch in diameter. The upper plate is fixed to the vafe, and revolves with it ; 

 the lower is fixed to the ftand. In the lower plate are inferted two metallic hooks, diame- 

 trically oppofite each other, at F and G. They are cemented into holes drilled in the edge 

 of the glafs, which is near two-tenths of an inch thick. In the upper plate are inferted in 

 the fame manner two fmall tails of the fine flatted wire ufed in making filver lace. Thefc 

 tails are bended down fo as to ftrike the hooks in the revolution, but in all other pofitions 

 they remain freely in the air without touching any part of the apparatus. At C is a fcrew, 

 which by raifing or lowering the vafe keeps the faces of the glafs plates from each other 

 at whatever diftance may be required. The faces of the glafs plates which are oppofed to 

 each other are coated with fegments of tin foil, as reprefented Fig. 9 and 10, the latter of 

 which reprefents the upper plate. Each of the tails communicates with the tin foil coat- 

 ing to which it is contiguous, as does alfo the hook F with that coating of the lower plate 

 neareft to it. But the hook G is entirely infulatcd from the whole apparatus, and is in- 

 tended to communicate only with the elearified body or atmofpherical conduaor L. 

 Vol.. I.— April 1797. P 1'J'<= 



