Development of a Specialized Hydrostatic Comparator for the Accurate Density Determination of Natural Silicon Spheres: A Novel Method for a Primary Realization of the Unit Kilogram

A novel method to primary realize the soon-to-be redefined unit of mass, kilogram, is presented. The new approach will accurately relate the Planck constant to spheres manufactured from natural silicon. Therefore, the precisely known isotopical composition of a 28Si sphere will be transferred to a sphere made of natural silicon with a relative uncertainty of 3 × 10−8. To achieve this objective, the concept of a newly developed hydrostatic comparator, which combines the Archimedes principle with a magnetic suspension coupling, is discussed. This combination allows to improve the major uncertainty contribution of this type of instrument - the temperature stability.