National Physical Laboratory

Watt balance anniversary

June 2015 sees the 40th anniversary since the principle of the watt balance was first proposed at the National Physical Laboratory (NPL) by Dr Bryan Kibble. The watt balance is one of the ways to measure mass in terms of a fundamental constant, which has led to the proposal to redefine the SI unit of mass, the kilogram, in 2018.

Watt balance
Mark II Watt balance

Mass is the only SI base unit that is still defined in terms of a physical artefact: the International Prototype of the kilogram (IPK) - a unique cylinder of platinum-iridium which is defined, within the SI, to have a mass of exactly one kilogram. The main problem with the definition is that the mass of the IPK could change due to contamination, wear or, however unlikely, loss or damage.

The search has been on to find a way of measuring mass in terms of a fundamental constant, leading to a definition of the kilogram in terms of such a constant. Two key approaches have been shown to be capable of making such measurements with the required accuracy: watt balances and the Avogadro project A watt balance compares measurements of electrical and mechanical power, giving a measurement of mass in terms of the Planck constant and other SI units.

Since the idea was first proposed, NPL has operated two watt balances, both of which were built by NPL scientists Bryan Kibble and Ian Robinson. The original (Mark I) balance operated in air and, in conjunction with SI measurements of the ohm, was used to determine the ampere in terms of the SI base units: the kilogram, the metre, and the second. This and other measurements led to an agreement in 1990 which united electrical measurements throughout the world. The second (Mark II) balance was designed to measure the Planck constant and pave the way for the redefinition of the kilogram. It operated in vacuum to eliminate the effects of the atmosphere on mass and velocity measurements and, by 2007, was producing high quality results.

In 2009, the NPL Mark II watt balance transferred to the National Research Council Institute for National Measurement Standards (NRC-INMS) in Ottawa, Canada. This move represented a unique opportunity to transfer a major metrological experiment to a new location and for an independent scientific team to take responsibility for future development of the experiment. The measurements made with the apparatus operating in Canada agreed with those made in the UK and the apparatus has produced the lowest uncertainty measurement of the Planck constant in the world to date. The result is in agreement with that of the Avogadro project and therefore underpins the effort to redefine the kilogram in terms of the Planck constant in 2018.

Watt balances operating at a number of laboratories around the world have contributed results to the redefinition effort and improved balances are being constructed in the USA, Switzerland, France, Korea, China, New Zealand and at the Bureau International des Poids et Mesures (BIPM) near Paris.

In 2014, Bryan Kibble and Ian Robinson published newly-discovered principles of watt balance design leading to a new generation of watt balances which can be far simpler to operate than their predecessors.

Find out more about watt balances

Find out more about redefining the kilogram

2014 was the 125th anniversary of the kilogram

Watch Bryan Kibble, Tony Hartland and Ian Robinson talking about the watt balance and redefining the kilogram

For more information, contact Ian Robinson

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Last Updated: 23 Jun 2015
Created: 23 Jun 2015


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