New Standards of Mass

In this chapter we discuss the efforts to redefine the unit of mass, the kilogram, and to develop a standard adequate to the new definition. The kilogram is likely to be redefined in relation to Planck’s constant. The work in progress discussed in this chapter aims to determine Planck’s constant by means of a watt balance and the Avogadro constant by counting the number of atoms in a mass standard of monocrystalline silicon 28Si (XRCD method). For the new definition to be adopted the uncertainty of the results of measurements performed by three different teams should be better than 5 × 10−8 and the results obtained by different methods should agree. Apart from the two methods mentioned above, we discuss the development of a mass standard based on the accumulation of heavy metal ions of gold (197Au) and bismuth (209Bi).