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The re are se ve ral s implifica tions whi ch c an be made in t he performance of the hydrostatic experime nt wh ic h le ad to more re produc ibl e res ults. Th e operation of s in gle-pan two-knife balances is di sc ussed , a nd s imp le mod ifi c at ions suggested to adapt it to h ydros tati c work _ A data philosophy is Ilrese nted wh ic h will be partic ularly a pplica bl e to a generation of now-d evelopin g forc e measuring ms trum e nts whe n used in hydros tati c work. A s im pli fi ed form ul a for a ir de nsity is prese nted a nd a formu la for es timat in g day-to-day variabi lit y in th e d e ns ity of wate r. A simpl e ba la nce ca lib ration procedure is prese nted in a n appendi x, and simpl ifi ed me th od s of fabri catin g s us pen sio n wires a nd degass in g sa mple s urfaces are d escribe d. T he use of th ese tec hniqu es is ill us trated by mea s ure me nts on s ilico n c rysla ls wh ic h indi cate process re prod uc ibilil y of a standard d e viati o'ri to a bo ut one part per mill ion.Key Word s: Air den s it y, ba la nce, bal a nce c ali bration , ba la nce e quilibrium , ba la nce se ns iti vi ty, de ns ity , double s ubs tituti on we ighin g, hydrostatic ba la nce, hydros tati c we ighin g, s ilicon d e ns ity , s ub s titution ba lance , volum e tri c meas ure me nt , water de ns it y, wate r we ig hin g.

We h ave dete rmin ed th e de ns ity of four pi eces of s in gle crystal s ili co n in te rm s of unive rsa lly ac ce pted s tand a rd s of mass and length. Th ese four obj ects will be use d as a workin g de ns ity stand a rd to whi c h a ll future den s it y work in th e Unit ed S tates will be re fe rre d. Us in g th ese c rys tal s as s t a ndard s, NBS ca n ca librate oth e r obj ec ts as de ns it y sta nd a rd s for assoc iated labora tori es.Th e work was acco mpli s hed with th e ass ista nce of a n interferometer, de veloped especia ll y for th e task , whi ch meas ures th e diam e te r of co mm ercia ll y ava ilabl e s tee l balls. From meas ured diamete rs ba ll volum es a re ca lc ul a ted. Th e volum etri c information co nt ain ed in th e balls is tra nsfe rre d to the s ilico n c rystal s in a ne wl y d es ign ed hydros ta ti c wei ghin g e xpe rim e nt.We have ma de three ind e pe nd e nt de nsity d ete rmin a tion s on eac h of th e four c rys ta ls, a nd th e prese ntly acce pted valu es of th e d ens ity of ea ch c rys ta l is th e average of the three de ter mination s. Th e ra ndom component of unce rtainty (3 s ta nd a rd de viation s) of th ese four ave rages is 0. 7 pplll. Th e syste ma ti c e rror is es tim a te d to be about 0.7 ppm.Key word s : De nsity s ta nd a rd ; hydros tati c we ighin g; perfect sp he re; sili co n; s ph e ri ca l inte rfe rom e te r; s ph e ri c al volum e ; volum e s tand a rd. Content s Page

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