This tabletop device turns the quantum definition of a kilogram into a real mass

The mini Kibble balance will measure 10 grams to a few ten-thousandths of a percent accuracy

Kibble balance

WEIGH IN  Small enough to fit on a table, this mini Kibble balance measures smaller units of mass, such as a few grams, to an accuracy of a few ten-thousandths of a percent. A Lego figurine (top right) holds a 1-gram mass for scale.

Leon Chao/NIST

It’s mass for the masses.

A tabletop device makes the new definition of the kilogram more accessible. Previously, the kilogram had been equal to the mass of a special metal cylinder kept in a vault near Paris. But researchers did away with that standard on May 20, pegging the kilogram instead to a quantum mechanical number known as the Planck constant (SN Online: 5/20/19).

Using that new definition, scientists at the National Institute of Standards and Technology in Gaithersburg, Md., created a scaled-down version of a device called a Kibble balance to directly measure masses of several grams via the Planck constant. When the prototype’s kinks are worked out, the apparatus should be accurate to a few ten-thousandths of a percent, researchers report in the June IEEE Transactions on Instrumentation and Measurement.

A full-scale Kibble balance requires its own laboratory space, costs millions of dollars to build and demands Ph.D.s to run it. But the new, suitcase-sized Kibble balance is just over half a meter tall, with a price tag around $50,000. That puts it within reach for pharmaceutical companies, for example, which must accurately dole out small drug dosages.

Traditional balances work by comparing the weights of masses in two different pans. But a Kibble balance compares a mass to the electromagnetic force needed to hold up that mass. Certain electromagnetic quantities, like voltage and resistance, can be tied back to quantum measurements involving the Planck constant, connecting that quantity to the object’s mass.

Because the Planck constant has the same value everywhere, researchers can directly measure out masses anywhere and anytime, without referencing the Parisian artifact.

Fun times with Legos inspired the new instrument. The researchers previously had made a Lego Kibble balance to help teach the public how the instruments work, says NIST mechanical engineer Leon Chao. That experience “subconsciously planted a seed.”

Physics writer Emily Conover has a Ph.D. in physics from the University of Chicago. She is a two-time winner of the D.C. Science Writers’ Association Newsbrief award.

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