Scientists redefine kilogramme in terms of Planck's constant

New kilogramme measurement of Planck's constant is 6.626069934 x 10-34 kilogrammes square mps

Scientists have made the most precise determination yet of the Planck's constant - an important value in science that will help redefine kilogramme, the official unit of mass in the international system of units.

Using a state-of-the-art device for measuring mass, researchers at the US National Institute of Standards and Technology (NIST) found that the new measurement of Planck's constant is 6.626069934 x 10-34 kilogrammes square metre per second, with an uncertainty of only 13 parts per billion.

The previous measurement had an uncertainty of 34 parts per billion.

The kilogramme is currently defined in terms of the mass of a platinum-iridium artefact stored in France.

Scientists want to replace this physical artifact with a more reproducible definition for the kilogramme that is based on fundamental constants of nature.

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, which uses electromagnetic forces to balance a kilogramme 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 kilogramme 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.

There are three major reasons for the improvement in the new measurements, said NIST physicist Stephan Schlamminger, who led the effort.

The new result uses 16 months' worth of measurements, from December 2015 to April 2017. The increase in experimental statistics greatly reduced the uncertainty in their Planck value.

The researchers also tested for variations in the magnetic field during both modes of operation and discovered they had been overestimating the impact the coil's magnetic field was having on the permanent magnetic field.

Their subsequent adjustment in their new measurements both increased their value of Planck's constant and reduced the uncertainty in their measurement.

Finally, the researchers studied in great detail how the velocity of the moving coil affected the voltage.

"We varied the speed that we moved the coil through the magnetic field, from 0.5 to 2 millimetres per second," said Darine Haddad, from NIST, lead author of the study that will be published in the journal Metrologia.