End of the kilogram as we know it

Scientists are drawing ever closer to redefining the kilogram, though it may be some time yet before the actual change is made. Research that affects how the kilogram – the last SI unit still defined by a material artefact -  will be defined in the future is being discussed this week at the Royal Society, the UK’s national academy of science.

The International System of Units (SI) is the world’s most widely used system of measurement units for commerce and science. It is established on seven base units (metre, kilogram, second, ampere, kelvin, mole, candela), from which all other units are derived.

Ideally, the base units should be stable over time and be universally accessible. This is not true of the kilogram which is the only base unit still defined by a manufactured object – the international prototype kept at the Bureau International des Poids et Mesures (BIPM) in France since 1889. The BIPM is the international home of metrology, the science of measurement. Measurements made over more than 100 years indicate that the mass of the international prototype may have changed by approximately 50 μg – this is the mass of a small grain of sand 0.4 mm in diameter – in comparison to the mass of an atom, such a carbon 12 or silicon 28 for example.  We believe the masses of atoms to be truly invariant quantities, constants of nature.

Scientists are conducting experiments so that the definition of the kilogram can soon be based on a fixed numerical value of a fundamental constant. Several constants could be chosen for this purpose but as it presents particular advantages to electrical metrology, the Planck constant – the fundamental constant of quantum physics – has been selected. As in previous redefinitions in the SI, the continuity will be maintained by choosing the size of the new unit to be the same as at present.

Dr Michael Stock, of the BIPM, will discuss on-going watt balance experiments as part of the Royal Society’s meeting. Watt balances establish a link between a mass and the Planck constant by comparing measurements of electrical and mechanical power. The electrical quantities are measured using macroscopic quantum effects, the Josephson effect and the quantum Hall effect. These two effects link the electrical quantities to fundamental constants and allow us to establish a relation between a macroscopic mass, measured in the unit kilogram, and the Planck constant.

Dr Stock says of results and progress so far:

“International consensus has been achieved, that in the near future the kilogram shall be redefined, based on a fixed value of the Planck constant. Our experiments are moving forward, however, it is too early to implement the new definition of the kilogram just yet. Experts in mass metrology recommend that until there is agreement among the results of experiments being run in laboratories across the world, a  new definition of the kilogram based on Planck’s constant will not be implemented. Another concern is that we are going to need more operating watt balances than are presently available for the robust realisation of the kilogram after the redefinition has taken place.”

According to the Royal Society’s Head of Library and Archives, Keith Moore, the society has a longstanding interest in the science of weights and measures. In 1743, “Gentlemen of the Society” checked and compared standard yards and pounds kept in the Guildhall, the Tower of London and elsewhere, to be used by clockmakers and other trades. Such work was vital to scientists too. In the 1760s for example, Charles Mason and Jeremiah Dixon used a ‘brass statute yard’ in the course of surveying State lines in what became the United States of America.

Keith Moore says:

“By the 19th century, the Royal Society kept copies of the Parliamentary standard yard (1853) and pound (1855). This duplication was essential: many original English weights and measures were destroyed by fire with the Houses of Parliament in 1834, an event itself caused by the burning of old counting devices called tally sticks. The 1850s standard measures can still be seen at the Royal Society. 

By the 1890s, later-Royal Society President Lord Rayleigh led the Treasury committee which recommended the establishment of the National Physical Laboratory, charged with the scientific determination of physical constants. “