Possible new definitions of the kilogram

The kilogram is the unit of mass in the International System. Its definition dates from 1889 and, therefore, predates most of modern physics. The definition simply states that the 'kilogram' is the mass of an object known as the international prototype. Thus, we have the extraordinary situation that certain fundamental physical constants (the Planck constant, the Newtonian constant of gravitation, the mass of an electron, the mass of an atom of carbon-12, etc.) are measured in terms of an artefact that was manufactured in the nineteenth century. We begin by describing what is presently known about the long-term stability of artefact standards and how this stability can be experimentally monitored with respect to fundamental constants of physics. Finally, we suggest that present experiments are close to achieving the target uncertainty of the order 10(-8), although the current incoherence between two classes of experiments is cause for concern.