Using a state-of-the-art device for measuring mass, researchers at the National Institute of Standards and Technology (NIST) have made their most precise determination yet 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. Accepted for publication (link is external) in the journal Metrologia, these new results come ahead of a July 1 international deadline for measurements that aim to redefine the entire SI in terms of fundamental constants of nature.
Using a state-of-the-art device for measuring mass, researchers at the National Institute of Standards and Technology (NIST) have made their most precise determination yet 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. Accepted for publication (link is external) in the journal Metrologia, these new results come ahead of a July 1 international deadline for measurements that aim to redefine the entire SI in terms of fundamental constants of nature.
The new NIST measurement of Planck's constant is 6.626069934 x 10-34 kg-m2/s, with an uncertainty of only 13 parts per billion. NIST's previous measurement, published in 2016, had an uncertainty of 34 parts per billion.
The kilogram is currently defined in terms of the mass of a platinum-iridium artifact stored in France. Scientists want to replace this physical artifact with a more reproducible definition for the kilogram 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, originally called the watt balance. Physicists widely adopted the new name last year to honor the late British physicist Bryan Kibble, who invented the technique more than 40 years ago.
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Image: In measuring Planck’s constant with the NIST Kibble balance, researchers carefully measured the effects of the magnetic field that is generated to counteract the weight of masses. In their experiments, they varied the mass from half a kilogram to two kilograms. In this image, NIST kilogram K85 lays on top of NIST K104 for the two kilogram measurement. (Credit: NIST)
