Watt Balance - Measurements

Measurements

Accurate measurements of electric current and potential difference are made in conventional electrical units (rather than SI units), which are based on fixed "conventional values" of the Josephson constant and the von Klitzing constant, K_J–90 and R_K–90 respectively. The current watt balance experiments are equivalent to measuring the value of the conventional watt in SI units. From the definition of the conventional watt, this is equivalent to measuring the value of the product K_J2R_K in SI units instead of its fixed value in conventional electrical units.

The importance of such measurements is that they are also a direct measurement of the Planck constant h:

The principle of the "electronic kilogram" would be to define the value of the Planck constant in the same way that the meter is defined by the speed of light. In this case, the electric current and the potential difference would be measured in SI units, and the watt balance would become an instrument to measure mass.

Any laboratory which had invested the (very considerable) time and money in a working watt balance would be able to measure masses to the same accuracy as they currently measure the Planck constant.

In addition to measuring U times I, the laboratory must also measure v and g (the velocity and the local gravitational acceleration,) using experimental methods that do not depend on the definition of mass. The overall precision of m depends on the precisions of the measurements of U, I, v and g. Since there are already methods of measuring v and g to very high precision, the uncertainty of the mass measurement is dominated by the measurements for U times I, which is the value measured by the watt balance.