Stephan Schlamminger

The Planck constant

The Planck constant is closely related to the watt balance. The watt balance is one experiment that can determine the Planck constant with high accuracy. An often asked question in this context is: Why do we need a mechanical contraption to measure a constant that arises in quantum mechanics. Surprisingly the answer can be found in the units. The unit of h is Js or kgm2/s. In order to measure h the unit of mass, kg, is needed. This makes the experiment large.

The graph below shows recent measurements of h. The data shoiwn as green circles were measured with watt balances. The data shown as red squares were obtained by measuring Avogadro's number. The product of Avogadro's number and the Planck constant is well known from precise measurements of the Rydberg constant.

recent h measurements

Abreviations of NMIs
NRC National Research Council Canada
IAC International Avogadro Collaboration
NIST National Institute of Standards and Technology United States of America