In a brightly lit subterranean lab at the National Institute of Standards and Technology (NIST) sits a room-sized electromechanical machine called the NIST-4 Kibble balance.

The instrument can already measure the mass of objects of roughly 1 kilogram, about as heavy as a quart of milk, as accurately as any device in the world. But now, NIST researchers have further improved their Kibble balance’s performance by adding to it a custom-built device that provides an exact definition of electrical resistance. The device is called the quantum Hall array resistance standard (QHARS), and it consists of a set of several smaller devices that use a quirk of quantum physics to generate extremely precise amounts of electrical resistance. The researchers describe their work in a Nature Communications paper.

The improvement should help scientists use their balances to measure masses smaller than 1 kilogram with high accuracy, something no other Kibble balance has done before.

NIST-4 measurements were used to help scientists redefine the kilogram, the fundamental unit of mass in the International System of Units (SI), in 2019. Everything that must be weighed, from market produce to the ingredients in your cold medicine, relies on this new definition of mass.

The new custom-built QHARS device is an example of a measurement standard — an object or instrument that has some predefined relationship to a physical quantity such as length or time or brightness. The standard in this case is an electrical device that uses quantum principles to generate a precise amount of electrical resistance. This generated resistance then serves as a reference during the Kibble balance’s operation.

Original source can be found here.