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.
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NBS has measured the densities of four single crystal silicon objects to a standard deviation of 0.26 ppm with a systematic error of 0.7 ppm. These objects will be used as working standards to which density work in the United States will be referred. A redundant hydrostatic comparison test involving two standards at a time has been devised, the use of which facilitates the efficient propagation of the density information contained in these crystals into other objects which may be used as standards by other laboratories. This article describes a method of employing this redundant comparison procedure.
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