Progress towards the redefinition of the kilogram: Measurements of Planck’s constant using watt balances

The watt balance compares electrical power and mechanical power to determine Planck's constant h in SI units. If the definition of the kilogram in the SI is altered to fix the value of Planck's constant this will remove the last artefact standard from the SI and the watt balance will provide a way to realise the SI mass unit. At present there are three operational watt balances, with more planned or under construction. Two laboratories, NIST and NPL, have published results from their experiments. These results differ significantly and further work is required to discover the source of the discrepancy.