National Physical Laboratory

In memory of Dr Bryan Kibble, 1938-2016

It is with great sadness that we announce that Dr Bryan Peter Kibble passed away on Thursday 28 April 2016.

In memory of Dr Bryan Kibble, 1936-2016

Dr Kibble worked at the National Physical Laboratory (NPL) from 1967 to 1998 as an experimental physicist, and was made an NPL Individual Merit Fellow in 1985. He was instrumental in reshaping the International System of Units (SI), and is best known for his conception of the watt balance, one of the measurement approaches proposed for the redefinition of the kilogram. Dr Kibble will be dearly missed by the international measurement community, with former colleagues citing his quiet and patient guidance, and praising his problem-solving skills.

Before NPL

Dr Kibble was born in 1938 in Berkshire. He studied Physics at Jesus College, University of Oxford, where he was awarded a DPhil in 1964 for research in atomic spectroscopy. Dr Kibble continued his research as a Postdoctoral Fellow at the University of Windsor in Ontario, Canada, from 1965 to 1967, before joining NPL as a Senior Research Fellow in 1967.

Early impact on the SI units

Early in his NPL career, Dr Kibble successfully measured the high field gyromagnetic ratio of the proton. This measurement, in conjunction with a similar low field measurement, indicated that there was a problem with the existing realisation of the ampere, the SI base unit of electrical current. At the time, the ampere was realised using the current balance, an instrument that was difficult to operate and that had a number of inherent limitations, which Dr Kibble was later to address.

Dr Kibble also worked with Dr Geoffrey Rayner on Coaxial AC Bridges and the calculable capacitor from which the SI definition of the unit of resistance, the ohm, could be established. In 1984, Dr Kibble and Dr Rayner compiled and published their research in the book, Coaxial AC Bridges

The imperfections of the current balance weighed on Dr Kibble's mind and inspired him to conceive a new and improved instrument, the moving coil watt balance, which, together with the calculable capacitor realisation of the ohm, could replace the current balance.

The invention of the watt balance

In the early 1970s, Dr Kibble had an idea for a measurement device that would supersede the current balance. He described his idea to Bob Cutkosky, a highly-respected experimental scientist visiting from the USA's National Institute of Standards and Technology (NIST). Cutkosky's response encouraged Kibble to proceed with the idea and also planted the seeds for similar developments in the USA, which were pursued by his friends Ed Williams and Tom Olsen.

In 1978, the Mark I watt balance was built at NPL with Dr Ian Robinson and Ray Smith. The instrument was used to realise the ampere with greater accuracy than was possible with the current balance, and the results played a major role in setting the 1990 conventional values of the Josephson and von Klitzing constants, used today for electrical measurements throughout the world. In recognition of his work, Dr Kibble was awarded the International Union of Pure and Applied Physics SUNAMCO Senior Scientist Medal in 1992.

Redefining the kilogram

The kilogram, the SI base unit of mass, is the last of the seven SI base units to be defined by a physical object. But in 2018, the kilogram will be redefined in terms of a natural constant, the Planck constant, the quantum of action in quantum physics.

In 1990, a second watt balance was built by Dr Kibble, Ian Robinson and Janet Belliss at NPL. It was designed to operate in a vacuum and was intended to measure the Planck constant with sufficient accuracy to support the redefinition of the kilogram.

The watt balance compares the weight of a one kilogram mass to the electromagnetic force generated by the interaction of a current-carrying coil of wire and a magnetic field. Then, the same coil is moved with a measured velocity in the same field, and generates a measured voltage. The combination of these two parts enables the properties of the coil and magnet to be eliminated from the measurement and allows electrical power and mechanical power to be equated. Using the Josephson and quantum Hall effects, electrical power can be measured in terms of the Planck constant and time, allowing the watt balance to relate mass to the Planck constant and SI units of length and time. By changing the definition of the unit of mass within the SI to fix the value of the Planck constant, the last artefact standard in the SI – the platinum-iridium cylinder kept at the International Bureau of Weights and Measures (BIPM) in Paris - can be replaced and, by the additional fixing of the value of the elementary charge, the electrical units can return to the SI.

In 2014, Dr Kibble and Dr Robinson published new principles for building simple watt balance designs, making the instrument more accessible, and in 2014, Canada's National Research Council used the NPL Mark II watt balance to measure the Planck constant with sufficient accuracy for the redefinition. A fitting tribute to Dr Kibble's visionary work, from 2018 watt balances should be used throughout the world to realise the kilogram definition.

An active retirement

Dr Kibble retired from NPL in 1998, but continued to be active in the field. He worked on the Mark II watt balance and high-frequency standards and bridges at NPL, and became a guest worker at both the Physikalisch-Technische Bundesanstalt (PTB) and BIPM, where he played a key part in eliminating a number of unresolved problems with the measurement of the ac quantum Hall effect.

Dr Kibble continued to be active on various international committees. In 2009, he won the IEEE Joseph F Keithley Award in Instrumentation and Measurement and was invited to write a regular column for IEEE Instrumentation and Measurement Magazine. In 2010, he published a book with Jurgen Schurr and Shakil Awan, Coaxial Electrical Circuits for Interference-Free Measurements

Dr Kibble gave his last talk at NPL on 17 March 2016, describing the invention and development of the watt balance to an audience of current and retired NPL staff, and members of the Institute of Physics and the British Society for the History of Science.

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Last Updated: 16 May 2016
Created: 6 May 2016

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