A Density Scale Based on Solid Objects

We have determined the density of four pieces of single crystal silicon in terms of universally accepted standards of mass and length. These four objects will be used as a working density standard to which all future density work in the United States will be referred. Using these crystals as standards, NBS can calibrate other objects as density standards for associated laboratories. The work was accomplished with the assistance of an interferometer, developed especially for the task, which measures the diameter of commercially available steel balls. From measured diameters ball volumes are calculated. The volumetric information contained in the balls is transferred to the silicon crystals in a newly designed hydrostatic weighing experiment. We have made three independent density determinations on each of the four crystals, and the presently accepted values of the density of each crystal is the average of the three determinations. The random component of uncertainty (3 standard deviations) of these four averages is 0.7 ppm. The systematic error is estimated to be about 0.7 ppm.