Kilogram will get more accurate in 2019. Researchers at the National Institute of Standards and Technology (NIST) have made their most precise determination of Planck's constant, an important value in science that will help to redefine the kilogram, the official unit of mass in the SI, or international system of units.
For 125 years, a salt-shaker-sized cylinder housed at the Bureau International des Poids et Mesures (BIPM), Paris and weighing exactly a kilogram served as the definition of the measure.
Scientists want to replace this physical artefact with a more reproducible definition for the kilogram that is based on fundamental constants of nature. India’s National Physical Laboratory too has a replica of this, since 1957, and it has served as the reference for a variety of industries to keep their weights accurate.
Planck's constant enables researchers to relate mass to electromagnetic energy. To measure Planck's constant, NIST uses an instrument known as the Kibble balance, originally called the watt balance. Physicists widely adopted the new name last year to honour the late British physicist Bryan Kibble, who invented the technique more than 40 years ago.
NIST's Kibble balance uses electromagnetic forces to balance a kilogram mass. The electromagnetic forces are provided by a coil of wire sandwiched between two permanent magnets. The Kibble balance has two modes of operation. In one mode, an electrical current goes through the coil, generating a magnetic field that interacts with the permanent magnetic field and creates an upward force to balance the kilogram mass. In the other mode, the coil is lifted at a constant velocity. This upward motion induces a voltage in the coil that is proportional to the strength of the magnetic field. By measuring the current, the voltage and the coil's velocity, researchers can calculate the Planck constant, which is proportional to the amount of electromagnetic energy needed to balance a mass.
No more artefacts
In essence, the units were freed from being defined on the basis of artefacts, as these being objects, were subject to wear and tear and sources of eventual error. The new artefacts, according to the International Committee for Weights and Measures, ought to derive from the constants of nature that are all interdependent.
These include constants such as the Planck constant — the ratio of the electromagnetic radiation from a photon to its frequency — and the charge of an electron.
Until this now, the kilogram was the only one among the units still pegged to a real object and now — after a formal vote in 2018 — the world is set to redefine the kilogram in terms of the Planck constant, the second and the metre.
The undoing of the cylinder has been the Kibble balance.
It is a set of scales, which uses the force produced by a current-carrying wire in a magnetic field to balance the weight of a mass.
Through this, accurate measures of the Planck constant — the fulcrum of several of the standard units — can be made.
“At the level of large objects nothing changes,” said Dinesh Aswal, Director, NPL and India’s representative at the conference, “But when measures at the level of micrograms need to be made such as in preparing drugs, these errors hugely matter.”
In the last 60 years, several standard units — the second, metre, ampere, Kelvin, mole, candela and, the kilogram — have all ceased to be defined by physical objects.
One metre, for instance, was a platinum-iridium bar of that measure. In 1960, the metre was defined as the distance travelled by light in vacuum in 1/299,792,458 seconds.