The kilogram was the last base unit to be defined by a physical object. Science and industry required a more accurate and stable way to measure mass, so the new definition of the kilogram is in terms of a fundamental constant of nature, which improves its long-term stability. It also eliminates the necessity for traceability to a single physical artefact, so the mass scale can theoretically be realised at any location and at any value, not just at 1 kilogram.
NPL has developed the Kibble balance, which balances gravitational force with an electromagnetic force.
The weight of a mass is balanced against the force generated by a current-carrying coil held in a magnetic field. The ratio of the force generated by the coil to the current passing through it is calibrated in a second phase of the experiment, which measures the voltage generated by the coil as it is moved at a measured velocity through the magnetic field. As the voltage and the current are measured using quantum electrical standards, the kilogram can be defined in terms of a fixed value of the Planck constant, h, plus the existing definitions of the metre and the second. The ultimate target is to measure a kilogram with an accuracy of a millionth of one percent, every time.
Find out about NPL's research in this area
More information on the SI unit definitions from BIPM