The kilogram was the final base unit to be defined by a physical object. Science and industry required a more accurate way to measure extreme weights, so the new definition of the kilogram is in terms of a fundamental constant of nature, which improves its long-term stability and eliminates the necessity for traceability to a single physical artefact and improves scalability.
NPL has developed the Kibble balance, which balances the gravitational force with an electromagnetic force. The electromagnetic force can be calculated in terms of h, the Planck constant. The ultimate target is to measure a kilogram with an accuracy of a millionth of one percent, every time.
The weight of a 1 kg mass is balanced against the electromagnetic force generated by a current-carrying coil hung 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 plus the existing definitions of the metre and the second.