The Planck constant (h) is a fundamental physical constant, meaning that it has the same value everywhere in the universe. As the quantum of electromagnetic action, it appears in most quantum mechanical equations.

In recent years, the value of these Planck constants has been determined in many laboratories worldwide using independent experiments with the best possible accuracy. Two experimentally different measuring principles were used.

The watt, or kibble balance was one way of determining the Planck constant. It links the mass and h by measuring an electric current. Alternatively, silicon spheres of single silicon crystals can be used to determine of h. This measuring principle links the mass of the silicon atom with the Planck constant.

Watt balance at NIST, National Metrology Institute of the USA