This device turns the kilogram’s new definition into a real mass

The tabletop system measures 10 grams to an accuracy of a few ten-thousandths of a percent

Small enough to fit on a table, this mini Kibble balance measures smaller units of mass, such as a few grams, with amazing accuracy. A Lego figurine perched on the top right holds a 1-gram mass for scale.

Leon Chao/NIST

Earlier this year, the kilogram got a new definition. With it, scientists can now measure mass very accurately with a type of scale that uses electromagnets. A new tabletop version of that device should now make accurate such measurements of mass more accessible to the masses.

Until this spring, the kilogram was defined as the mass of a special metal cylinder kept in a vault near Paris, France. But researchers did away with that standard on May 20. Now the kilogram is pegged instead to a fundamental constant of physics. That number is known as the Planck constant. (A fundamental constant is a number that doesn’t ever change.)

With the new definition, scientists can use a device called a Kibble balance to directly measure masses via the Planck constant. The device is named after Bryan Kibble, the physicist who invented it.

But a full-scale Kibble balance is extremely complex. It requires its own laboratory space and costs millions of dollars to build. And it needs a host of PhD-level scientists to run it.

Now a team at the National Institute of Standards and Technology in Gaithersburg, Md., has created a scaled-down version of the device. They are designing it to measure smaller masses, about 10 grams (0.35 ounce). When the prototype’s kinks are worked out, the apparatus should be accurate to a few ten-thousandths of a percent. The researchers described the tabletop device in the June IEEE Transactions on Instrumentation and Measurement.

The new, suitcase-sized Kibble balance is just over half a meter (1.6 feet) tall, with a price tag of around $50,000. That puts it within reach of labs that frequently need to weigh small things. For example, companies must accurately dole out small doses of the drugs they make.

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 measurements, such as voltage and resistance, can be tied back to the Planck constant. That allows a series of equations to connect that quantity to the object’s mass.

The Planck constant has the same value everywhere. That means researchers can directly measure out masses anywhere and anytime. They no longer have to compare things against the Parisian cylinder.

Fun with Legos inspired the new instrument, says Leon Chao. He is a mechanical engineer on the team. The researchers had made a Lego Kibble balance to help teach the public how the instruments work, he says. That experience gave them the idea to make a small-scale real one.

This Lego model of a Kibble balance is at the National Institute of Standards and Technology (NIST). It helped inspire Leon Chao and others to build a real one at a similar scale.


Science News physics writer Emily Conover studied physics at the University of Chicago. She loves physics for its ability to reveal the secret rules about how stuff works, from tiny atoms to the vast cosmos.

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