The NPL moving-coil apparatus for measuring Planck's constant and monitoring the kilogram

In a moling coil balance, a coil threaded by a strong magnetic flux is moved with linear velocity u to induce a voltage V. In a separate measurement a force F caused by a current I (= V/R where R is a resistance) flowing in the coil is then weighed as a mass dl times gravitational acceleration g, The relationship M = V-2/Rgu is obtained by eliminating the rate of change of magnetic flux threading the coil, A relative uncertainty of some parts in 10(9) seems possible, enabling the stability of the kilogram to be monitored in terms of the Josephson effect (similar to h/2e) used to measure V and the quantum Hall effect (similar to h/e(2)) used to measure R, Therefore M = Ah where A is a measured quantity involving only meters and seconds, and either Planck's constant h could be measured in terms of the present kilogram or the kilogram could be redefined in terms of a defined value of ii. We report progress with the NPL apparatus toward these ends.