 Currently there are over 200 atomic clocks in over 50 national laboratories around the world and the BIPM, in terms of the mandate given to it under the Metre Convention, coordinates the various atomic clocks.After 1960, when the definition of the metre was linked to a particular wavelength of light rather than the international prototype of the metre, the only unit of measure that remained dependent on a particular artefact was the kilogram.Over the years, small drifts which could be as high as kilograms per annum in the mass of the international prototype of the kilogram were detected.