Alas, poor kilogram!

The quadrennial General Conference on Weights and Measures (GCWM) doesn’t usually make the evening news, but it got a mention on Friday 16 November when it redefined the definition of four commonly used units of measurement, including the kilogram. Also affected were the ampere, kelvin and mole, so that altogether four of the seven measurement units that comprise the International System of Units (SI) were changed.

Advanced measuring instruments like the NIST-4 Kibble balance have enabled a redefinition of SI units like the kilogram.

Instead of linking the SI units to physical artefacts, the revision ties their values to fundamental physical constants that allow for far more precise measurements. Since 1889, the kilogram, for example, was linked to the weight of a platinum/iridium compound cylinder referred to as the International Prototype of the Kilogram (IPK), colloquially known as Le Grande K. While the original was carefully housed in a sealed glass container in a suburb of Paris, copies of the IPK were distributed to measurement associations around the world.

Eventually it was discovered that there were miniscule variations in weight between the copies and original. The physical substance of the IPK was also subject to natural degrading processes, resulting in extremely small fluctuations in weight, as small as 50 parts in a billion. While these are not significant fluctuations if you’re weighing meat at the butcher’s, they become extremely important if you are, for example, conducting experiments at a subatomic level.

Fundamental forces of nature remain consistent and can be measured far more precisely, hence the GCWM’s decision to adopt the following as the constants underlying SI measurements:

• The kilogram will be linked to Planck’s constant. The Planck constant relates alight particles energy, and hence mass, to its frequency. New measuring technology like NIST-4 Kibble balance can measure the constant with incredible accuracy.

• The kelvin will be linked to the Boltzmann constant. This constant relates temperature to energy by means of statistical mechanics. Previously, the kelvin was measured as a fraction of the triple point of water, but its not possible to prepare two exactly identical formulations of the isotopic mixture of water required for this test. New advances in acoustic thermometry have resulted in extremely accurate measurements of the Boltzmann constant.

• The ampere will now be measured according to the elementary electrical charge. Where the past definition relied on the charge in newtons between two wires containing 1 ampere each and separated by a distance of 1 metre, the new definition is refreshingly straightforward: 1 ampere equals 1 coulomb per second.

• The mole will be linked to the Avogadro constant. In the present metric system, a mole is the amount of substance that contains as many elementary entities as there are atoms in 0.012 kilograms of carbon-12. The Avogadro constant is expressed as the number of molecules in one gram-molecule of oxygen.

The new defining constants of the International System of Units

Defining constant Symbol Numerical value Unit

Hyperfine transition frequency of Cs ∆vCs 9 192 631 770 Hz

Speed of light in a vacuum c 299 792 458 m s -1

Planck constant h 6.626 070 15 x 10 -34 J Hz -1

Elementary charge e 1.602 176 634 x 10 -19 C

Boltzmann constant k 1.380 649 x 10 -23 J K -1

Avogadro constant NA 6.022 140 76 x 10 23 mol -1

Luminous efficacy K cd 683 lm W -1