The 1991 paper written by the BIPM Director of the day called upon the community of measurement experts to find in due course a way of comparing the mass of the International Prototype to a fundamental constant of physics. This would be the only sure way to resolve the following question: is the International Prototype getting lighter, or are most of the copies getting heavier, or are none of these objects stable in mass with respect to the fundamental constants? The suggestion was that the link between the present kilogram unit and a fundamental constant should be made to an accuracy of 20 parts per billion (20 micrograms in one kilogram). The challenge to the measurement community was made more formally in a 1995 Resolution of the General Conference for Weights and Measures. Since 1995, a second and distinct reason for doing this work has been generally accepted: the kilogram is the last remaining base unit still defined by an artefact rather than by a fundamental constant of nature. As a consequence, the values of some fundamental constants (Planck constant, elementary charge, Avogadro constant, etc.) are currently measured in terms of the International Prototype, which has nothing fundamental about it. For this reason, many in the scientific community see an immediate benefit to defining the unit of mass, the kilogram, in terms of a fundamental constant of physics. The General Conference for Weights and Measures has reviewed this situation at its meetings in 2011 and 2014 and it is now planned to redefine the kilogram in the near future, together with the ampere, the kelvin and the mole.

But this redefinition will only be practical for the mass community if the present kilogram can be linked to the chosen constant with sufficient accuracy. Thus the linking experiments are very important. They are also very difficult.

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