VERY SENSIBLE MICRO-ELECTROMETER. JSJ. 



The glafs cylinder has, about I" (centimetre, near 4 lines Throtrgh abt^ 

 Englifh meafure?) from the plate on which it is fixed, a fmall ^"microme'tcr- 

 hole, through which pafles the extremity of a micrometer- fcrewpaffK, 

 ferew, about the iize of a large goore-quiil, and very care- 

 fully cut. This fere w has fifty threads in a Rhynland inch, 

 cut very deep, though very fine. Il is made of two pieces, 

 and fliouid be at leaft three quarters of an inch long, to avoid 

 any ftiake Tiie extremity of this fcrew carries a little ball, 

 wliich is put on after the fcrew is pafl'ed through the opening 

 in the cylinder. To avoid all fi-iction againft the glafs, care 

 is taken that the fcrew, when turned, does not touch the 

 edges of the cylinder. The fcrew carries a plate three inches 

 and a half (3.8 Englifli) in diameter, which has 360 divi- 

 iions, and confequently divides each thread of the fcrew into 

 as many parts. 



In this manner we are enabled to determine the fphere of l^y which the 

 aaivity of the two eledricities in IHOOOlhs of a Rhynland ^^^""^^^ 

 inch. The mounting which contains the female fcrew has a ,nch may be 

 fmall pillar, which advances on the plate, and carries" an in- nieaiu:ed. 

 dex, by which the degrees are marked with precifion. 



To ufe this inftrument, which is perfectly infulated by the Mo^e of prc- 

 glafs plate on which it refls, the firft thing is to place the plate ftrunfentfor"ufc. 

 in fuch a pofition that the fhall be exadlly under the index. 

 The ^djufting fcrew which carries the nippers is then to be 

 moved till the leaf filvcr is fo near the ball, that no light pafles 

 between them. Thus we have the point of contafl, and of 

 for the fphere of a6tivity of the two eledricities. To be certain 

 that the leaf is brought as clofe as poflible to the ball without 

 being forced out of the vertical diredlon, the micrometer fcrew 

 fliould be moved a turn firft backward then forward feveral 

 times, and the pofition of the leaf obferved every time the 

 ball is brought into conta6t with it. The inftrument being thus 

 adjufted, the micrometer- fcrew muft be moved backward one 

 turn, and then we have between the leaf and the ball a diftance 

 of one-fiftieth of an inch, which may be fubdivided at pleafure 

 by means of the plate'; for with a plate near four inches in 

 diameter, and by means of the fine needle on which it turns, 

 we may diftinguiQi half or even a quarter of a degree if 

 necelTary, 



An adjuftment fixed to the plate on which the cylinder refts Apparatus for 

 fefves to afcertain whether the leaf of filver be in fad drawn ascertaining the 



perpendicularity 

 out of the leaf. 



