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Organised by Dr Terry Quinn CBE FRS, Professor Ian Mills FRS and Professor Patrick Gill

From the origins of the metric system, when the metre was a fraction of the arc of the Paris meridian and the kilogram the weight of a cubic decimetre of water, the ultimate goal has been a system of measurement based on invariant quantities of nature. After more than 200 years we are now within reach of achieving this. While the kilogram is still defined as the mass of a Pt-Ir cylinder kept in a vault in Sèvres, serious plans now exist to redefine the kilogram by fixing the numerical value of the Planck constant h; and the ampere, kelvin and mole by fixed numerical values for e, k and NA. With the metre already being defined by the speed of light and the second by an atomic microwave transition, but likely soon to be redefined by an optical transition of much higher frequency, we shall have at last achieved what the savants of the 18th century had sought. The Discussion Meeting will review the relation of the International System of Units to the fundamental constants of physics and progress towards the redefinitions.

The biographies and audio files are available below.

The proceedings of this meeting are scheduled to be published in a future issue of Philosophical Transactions A.