The Herculean effort to define the kilogram

One year ago today, Dr Bryan Kibble, an esteemed scientist from the National Physical Laboratory best known for his work on redefining the kilogram, passed away. An experimental physicist, Dr Kibble was heavily involved in reshaping the International System of Units (SI).

In 2018, the kilogram – the SI base unit of mass and last physical unit characterised by an object ­– will be redefined based on the Planck constant. This has been made possible through use of the watt balance, an analytical device first created in the 1970s by Dr Kibble and colleagues at the National Physical Laboratory (NPL).

The watt balance compares the weight of a one kilogram mass to the electromagnetic force generated by the interaction of a current-carrying wire coil and magnetic field. The same coil is moved with a measureable velocity that produces a quantifiable voltage. This technique allows electrical power measured by the watt balance to relate mass to the Planck constant and SI units of length and time.

Changing the definition of the SI unit of mass in this way will mean the platinum-iridium cylinder kept at the Internal Bureau of Weights and Measures (BIPM) in Paris will no longer be required. In 2014, Canada’s National Research Council decided that the watt balance (much updated from the first iteration) was accurate to enough be used to redefine the kilogram.

Dr Kibble studied Physics at the University of Oxford, receiving a DPhil in 1964 for research in atomic spectroscopy before travelling to Canada for a postdoctoral fellowship in 1965. Upon his return to the UK, he assumed the role of Senior Research Fellow in 1967 before retiring in 1998. Upon the anniversary of Dr Kibble’s passing, the NPL and others remember the impact of his work on the international measurement community, with former colleagues praising his problem-solving skills and quiet, patient guidance.