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

In a moving-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 M times gravitational acceleration g. The relationship M=V/sup 2//Rgu is obtained by eliminating the rate of change of magnetic flux threading the coil. A relative uncertainty of some parts in 10/sup 9/ seems possible, enabling the stability of the kilogram to be monitored in terms of the Josephson effect (/spl sim/h/2c) used to measure V and the quantum Hall effect (/spl sim/h/e/sup 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 h. We report progress with the NPL apparatus toward these ends.