iiiiiiiiii UNIVERSITY OF CALIFORNIA AT LOS ANGELES SUPPLEMENT TO THE ENCYCLOPEDIA, OR DICTIONARY OF ARTS, SCIENCES, AND MISCELLANEOUS LITERATURE. IN THREE VOLUMES. Illustrated with Copperplates. NON IGNORO QU^ BONA SINT, FIERI MELIORA POSSE DOCTRINA, ET QU.E NGN OPTIMA, ALIQUO MODO ACUI TAMEN, ET CORRIGI POSSE. — CiCERO. VOL. II. ELE PHI ^|)ilatjelpi)ia : PRINTED Br BUDD AND B^RTR.iM, FOR TIIOiMAS DOBSON, AT THE STONE HOUSE, N" 41, SOUTH SECOND STREET. 1803. SUPPLEMENT TO T.HE AE5 ENCYCLOPAEDIA. E L E Elfarome- T^ LECTROMETER, is an inarument which mea- ''^'■- I J lures the quantity of ele>5lricity in any eleiflrified body. The moll common elecflrometers are defcribed in the article Electricity (EncycL), n" 27, and 182 — 233. A very valuable one is likewile defcribed in n° 85. of the article Electricity in this Supplement; but there are ftill two eleftrometers, of which we have hitherto given no account, though they are of fuch va- lue, that to pals them unnoticed would be unpardon- able. The firft, which is by much the moll accu- rate and delicate inflrument of the kind that we have leen, was invented by Mr Coulomb, and is adapted to afcertain the fmalled quantity of redundant electricity. The fecond is a late invention of Mr Cuthbertfun, the ingenious improver of the air-pump, and is employed only to meafure the charge of large jars and batteries. Electrometer, by Mr Coulomb of the Royal A- cademy of Sciences at Paris, defcribed in the Memoirs for 1785. Mr Coulomb had made fome experiments in exami- nation of Dr Hooke's theory of fprings " ut tenjlo ftc vis ;" and found, that it was furprifingly exad, in re- gard to the force neceifary for twilling eladic wires. Having fufpended a nicely turned metal cylinder by a fine wire in the direiftion of its axis, and having given it feveral turns, and left it to regain its natural pofilion, heobferved, that it performed all its revolution of un- tvv'illlng and twilling in times precifcly equal, whether ihefe oTcillations were of a few degrees, or confiiled of feveral revolutions. He thence concluded, that the force with which the wire endeavoured to regain its natural pofition was exactly proportional to its dillance from it. Engaged, foon after, by order from the Mi- nifter of Marine, in an examination of the phen^ mcna of the mariner's compals, he took this method of fuf- pcnding his needles, in order 10 obtain exafl me.ifures of the forces which caul'td them to deviate from the magnetic meridian. He made ibmc obfervations with needles I'o fulpendcd ; whicii are highly valuable to the philulbpher engaged in that liudy. When Iiis fuccefs in this relearch had fully gratilie J liis wifhes, he turned his thuughis 10 the exuinina'ion ot tlie law of electiic aiflion by the help of an ck Urometer liifpendcd in the fame manntr. It is con(lru(5leJ as loUovs : ABDC (li^. I.) repreiVnts a gl ifs cylinder, 12 inclies in dianictjr and in heiglit. This is covered by a gl.ifs plate fitted to it by a prcjciSing fillet on the un- der lurlace. This cover is pierced with two round SuppL. Vol. II. Plate XXVII. E L E holes of i|th inches in diameter. One of them / Is in Elcar<.me- the centre, and it receives the lower end of the gl.ifs icr. tube / />, of 24 inches height, which is fixed in the hole ''■^""'^^^ with a cement made of fealing-wax, or other eleflric fubftance. The top of this tube receives the brafs collar H ( fig. 2 . n° 3. ), bored truly cylindrical, and hav- ing a fmall flioulder, which rells on the top of the tube. This collar is fadtned with cement, and receives the hollow cylinder * (fig. 2. n" 2.), to which is joined the circular plate a b, divided on the edge into 360 degrees. It is alfo pierced with a round hole G in the centre, whichreceivesthecylindfical pin/' (fig. 2. n° i.), having s milled head b, and an index i 0, whofe point is bent down, fo as to mark the divifions on the circle a b. This pin turns lliffly in the hole G, and the cylinder * turns lleadily in the collar H. To the lower end of the centre pin is faftened a little pincer q, formed like the end of a port-crayon, and tightened by the ring q, fo as to hold fad the fufpenfion wire, the lower end of which is grafped by a (iinilar pincer Po (fig. 3.), tighten- ed by the ring