Measurement of the Planck Constant and the Revision of the SI

Stephan Schlamminger (Physical Measurement Laboratory, NIST)

A revision of our system of units, the SI, is currently discussed and may be implemented as early as 2018. The new SI is a logical extension of an argument made in 1983 when the meter was redefined to be based on the exact value of the speed of light. In the new SI all units will be derived from seven fundamental reference constants, thus replacing the seven base units of the current system.

For example, the unit of mass, the kilogram, is currently defined by an artifact called the International Prototype of the Kilogram (IPK). In the future we will be able to realize the unit of mass, not just at the kilogram level, from a fixed value of the Planck constant, which has units of kg m²/s.

One condition for redefinition is agreement between different measurements of the Planck constant. Currently two measurement strategies lead to values with relative uncertainties less than 100 parts per billion (ppb): (1) Avogadro's number can be determined by estimating the number of atoms in a well characterized crystal. From Avogadro's number h can be calculated using the Rydberg constant, which is known with much smaller uncertainty (2) A watt balance can be used to measure mechanical power in units of electrical power. Electrical power can be measured as the product of the Planck constant and two frequencies by utilizing the Josephson effect and the Quantum Hall effect.

In this presentation, the two ways to measure h will be explained. An emphasis will be given to the measurements that were performed at NIST in the last 2 decades.

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