Kibble balance

 
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The Kibble balance, also known as the watt balance, measures how much electricity it takes to balance a weight suspended in a magnetic field. The difference between this process and using, for example, water, is that the Kibble balance is tied to universal fundamental constants—principles that dictate how the universe works, in this case the Planck constant—, and so we can factor out such things as atmosphere and other environmental errors.

To compare that to an ordinary balance, the typical balance measures an unknown mass (e.g. you on your bathroom scale) using a scale that one hopes is accurately calibrated.

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Why is it important

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As unlikely as it seems, the current global standard against which all things are weighed is a cylinder of metal in a vault in a suburb of Paris. This small cylinder of platinum-iridium alloy—a little more than 39 millimetres tall and the same in diameter—is the International Prototype Kilogram, or IPK. It is carefully sealed inside three concentric glass jars lest its mass be altered by dust, damage or fingerprints. It has been removed from its enclosure only four times since it was introduced as the IPK in 1889—to compare the accuracy of the half-dozen official copies that exist.

Scientists are increasingly concerned about the IPK's longer-term accessibility and stability, because its mass is not easily reproducible and gradually changes with time. Of the three fundamental constants that are involved in the Kibble balance experiment, the Planck constant (h) is currently the only one whose value is not fixed by definition and hence, the Kibble balance provides an accurate measurement of h, whose value will redefine the kilogram.

Who is Dr. Bryan Kibble

Dr. Kibble (1938-2016) worked at the UK 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.

Meet the NRC's Kibble balance experts

Carlos Sanchez
Carlos Sanchez, Research Officer in Electrical measurements at the NRC
Institution National Research Council of Canada
Current research and projects Kibble Balance, Quantum Hall resistance, Calculable Capacitor and impedance.
Education PhD in Physics, U of Maryland
Awards NRC Research breakthrough of the year award, 2012
Key publications
  • 'A summary of the Planck constant determinations using the NRC Kibble balance'
  • 'A determination of Planck's constant using the NRC watt balance'
  • 'Elimination of mass exchange errors in the NRC watt balance'
Expertise Electrical measurements, superconductivity and cryogenics.
Richard Green
Richard Green, Team Leader in Mass Metrology at the NRC
Institution National Research Council of Canada
Current research and projects Mass Metrology, Fundamental Constants, Bridging the Air-Vacuum Gap
Research and project statements Redefinition of the SI, Measurement of Fundamental Constants
Education PhD in Physical Chemistry University of Ottawa, BSc Physics and Mathematical Statistics, Concordia
Key publications
  • 'A summary of the Planck constant determinations using the NRC Kibble balance'
  • 'A determination of Planck's constant using the NRC watt balance'
  • 'Air–vacuum transfer; establishing traceability to the new kilogram'
  • 'Determination of Surface Potential and Electrical Double-Layer Structure at the Aqueous Electrolyte-Nanoparticle Interface'
Expertise Mass Metrology, Vacuum Science, Surface Science
 
Barry Wood
Barry Wood, Research Officer in Electrical measurements at the NRC
Institution National Research Council of Canada
Current research and projects Kibble Balance, Calculable Capacitior, Josephson Voltage Standard
Education PhD in Physics, U of Toronto
Awards NRC Fellow, IEEE Fellow, Astin Award 2006 & 2007
Key publications
  • 'A summary of the Planck constant determinations using the NRC Kibble balance'
  • 'A determination of Planck's constant using the NRC watt balance'
  • 'Elimination of mass exchange errors in the NRC watt balance'
Expertise Electrical measurements, cryogenics, fundamental constants