The National Institute of Standards and Technology (NIST) has developed a new watt balance that will help redefine the International System of Units (SI) by defining mass in terms of a fixed value of the Planck constant (h). This would replace the current kilogram standard, which is difficult to transfer and drifts over time.
The relationship between mass and h is not immediately obvious, but can be determined using the equation E = hν, where E is energy and ν is frequency. NIST’s Jon Pratt explains this relationship in an essay published in the March 2014 issue of Measure, a journal by the National Conference of Standards Laboratories International (NCSLI).
The new watt balance, currently under assembly and testing, will operate in a vacuum and cover its final configuration. Once complete, it will help establish a new mass standard based on h, marking a significant shift from the current artifact-based kilogram standard.
Keywords: Planck constant, mass, joule second, SI, frequency
