 The measurements use highly polished spheres of silicon with a mass of one kilogram . Spheres are used to simplify the measurement of the size ( and hence the density ) and to minimize the effect of the oxide coating that inevitably forms on the surface . The first measurements used spheres of silicon with natural isotopic composition , and had a relative uncertainty of 3 @.@ 1 × 10 − 7 . These first results were also inconsistent with values of the Planck constant derived from watt balance measurements , although the source of the discrepancy is now believed to be known . 
