NIST’s Kibble balance: A Mass revolution

NIST engineer Leon Chao never dreamed his childhood fascination with LEGOs would fit together with his professional life. But a working LEGO model he and his team built to demonstrate a revolutionary and highly accurate way to measure mass not only captured the imagination of the public but highlighted NIST’s efforts to radically simplify, streamline and transform the way mass is calibrated in all 50 states.

On May 1, Chao described NIST’s role in the evolution of mass metrology as the featured speaker for the Office of Weights and Measures’ Info Hours.

OWM Info Hours: Revolutionizing Mass Calibrations with the Tabletop Kibble Balance

Although mass metrology is now linked to quantum mechanics, it traditionally relied on physical artifacts, Chao noted. From 1889 through 2019, the standard kilogram—the reference against which all mass measurements were compared—consisted of a cylinder of platinum-iridium stored under three bell jars inside a triple-locked vault housed at the Bureau International des Poids et Mesures (BIPM) just outside Paris. Copies of this International Prototype of the Kilogram (IPK) were distributed to countries around the world as secondary, or national, standards.

However, over the years, as researchers compared various copies of the IPK with the original in France, they discovered a divergence in mass values, Chao said. Although the discrepancies amounted to only a few hundred-thousandths of a gram, they nonetheless revealed an instability within a supposedly stable definition. Because of these discrepancies, Chao told his online audience, “scientists have been pondering ways to dethrone the IPK for almost as long as the IPK has existed.”

But it wasn’t until relatively recently, he added, that researchers had the technology and knowhow to replace the IPK with a highly accurate new standard tied to Planck’s constant, a fundamental constant of nature that plays a key role in the quantum world.

An old-fashioned balance acts like a seesaw, with the downward weight of an unknown, test mass on one side of the scale countered by the downward weight of a known mass on the other side. But in 1975, British scientist Brian Kibble conceptualized a new type of scale. Now known as the Kibble balance, the device counters the force exerted by the downward weight of a test mass by an upward electromagnetic force. The force is generated by current flowing through a coil immersed in an external magnetic field. Operated in this mode, known as the weighing mode, the Kibble balance acts like a modern mass comparator, with the electromagnetic force proportional to the current in the coil.

At this point, it might seem like the test mass has been weighed and the job completed. The counterbalancing electromagnetic force can be calculated from a familiar equation dating from the 19^th century: Force = IBL, where I is the current, B the magnetic flux density and L the length of wire in the coil. But there’s a problem. BL is extremely hard to measure directly with high accuracy.

That’s where the second operating mode of a Kibble balance, the velocity mode, comes into play, distinguishing it from all other balances, Chao noted. In this mode, the test mass is removed and the current applied to the coil is shut off. The coil then moves through the surrounding magnetic field at a carefully controlled, constant velocity, which generates a voltage in the coil. When information from the weighing and velocity modes are combined, the troublesome BL term drops out of the equations and the mass can be determined to high accuracy. If voltage, current, the coil velocity, and the acceleration due to gravity are all known, then the test mass can be weighed with unprecedented accuracy.

But wait, there’s more.

Scientists measure current in a Kibble balance by placing a resistor in the circuit. Physics Nobel prize winner Klaus von Klitzing had previously shown that resistance can only take on discrete, or quantized values, determined by Planck’s constant. Studies by another physics Nobel prize winner, Brian Josephson, demonstrated that voltage can also be measured in terms of Planck’s constant.

Given these links to Planck’s constant, the Kibble balance measures mass in a way that is directly traceable to a fundamental, unvarying quantity in nature. The NIST Kibble balance, dubbed NIST-4, resides in a basement laboratory on the Gaithersburg campus and stands about as tall as a walk-in closet.

As of May 20, 2019, any time someone wants to check the accuracy of a scale, the calibration is ultimately traceable to NIST-4. Currently, researchers and manufacturers must periodically send their calibration weights to NIST to calibrate them against the Kibble balance. But that might soon change.

Back in 2013, as Chao and his colleagues were building NIST-4, they assembled a miniature Lego version of the balance as a way to educate the public about mass measurements and the Kibble principle. Two years later, when the researchers were ready to publish their results, the American Journal of Physics featured their design on the cover of its November issue. It created a sensation.

People around the world, from middle schoolers to adults, began building their own versions of the miniature LEGO balance. Inspired by the public’s enthusiasm, Chao and his colleagues began work on a lab-grade tabletop Kibble balance that industry, academia and the government could use in their own laboratories.

In 2024, NIST delivered a prototype of its tabletop Kibble balance to the U.S. Army, marking the first time in the U.S. The instrument eliminates the need for the Army to send equipment to NIST for costly and lengthy mass measurements.

Chao envisions the day when every mass measurement laboratory in the country has a group of tabletop Kibble balances, collectively measuring a range of masses from 1 milligram (thousandth of a gram) to 1 kilogram (thousand grams).

Achieving that goal, said Chao, “is not a question of if but when.”