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Fundamental Metrology

Amongst MSL reserach projects are a few that have the potential to underpin major changes in the way the SI units are defined and realised, or not specific to just one area of measurement and of broad scientific interest.

Measurement of the gravitational constant, G

The Newtonian gravitational constant G relates the gravitational attraction between two bodies to their mass and separation. The value of G is not well known becuase of the difficult to measuring the small gravitational forces that can be produced in a laboratory in the presence of many other stray forces.

When we started our measurement in the early 1990s the uncertainty in the recommended value for G was almost 0.1 percent due to the wide scatter in results reported by different groups.  Our measurement used a torsion balance with electrostatic compensation to balance the applied gravitational forces. Over the next decade we made several series of measurements of G as we refined our apparatus and used different torsion fibres and materials for the attracting masses. Our final result, reported in 2003 was G = 6.67387(0.00027) × 10-11 m3kg-1s-2, which has a relative standard uncertainty of 0.00004%.

This result, along with those from other measurement groups, was included by the CODATA committee to produce the 2002 and 2006 recommended values for the gravitational constant with a relative standard uncertainty of 0.0001%.

For further information contact Mark Fitzgerald or Tim Armstrong


T R Armstrong and M P Fitzgerald, New Measurements of G Using the Measurement Standards Laboratory Torsion Balance, Phys. Rev. Lett., 91, 20, 201101-1,2003.

Measurement of Planck's constant, h

Most of the measures we rely on in commerce, in science and in our everyday are defined in terms of fundamental constants or invariants of nature.  The sole exception is the kilogram, which is still defined by a physical object.  

The mass of the platinum-iridium cylinder that has defined the kilogram for over 100 years is known to be unstable.  This is preventing us from improving our global measurement ability through recent scientific advances such as the discovery of quantum electrical phenomena.  The hunt is now on world-wide to find a fundamental constant as the basis for the kilogram.  MSL is researching the watt balance approach which links the kilogram to the Planck Constant.  A watt balance relates mechanical power to electrical power by comparing the gravitational force on a mass with the force on a current-carrying coil in a magnetic field.  This allows mass to be linked to the Planck constant via the Josephson and quantum Hall effects.

For further information contact Chris Sutton


C M Sutton, An oscillatory dynamic mode for a watt balance, Metrologia, 46  467–472, 2009 

Single-electron tunnelling

Recent progress in fabrication technologies of nanometre scaled structures have attracted substantial theoretical and experimental interest due to the combination of their interesting physics and the range of possible applications. Examples of the applications include computing devices, switches, charge detectors, as well as chemical and biological sensors. Because of the ability of some of these devices to transfer individual electrons with extraordinary accuracy they are of particular interest as future measurement standards, such as quantum current sources and capacitance standards. We study the theory of electron transport in devices of various materials with the aim of developing transfer-process models or devices with sufficient accuracy to enable their use in metrological applications.

For further information contact Vladimir Bubanja