In a quiet lab, a device the size of a microwave oven sits on a table. Inside, a coil glides smoothly up and down while a computer reads electrical signals. This is the tabletop Kibble balance — a machine that links mass to quantum constants.
Previously, such balances occupied a whole room and required a team of engineers. Now, engineers at Tsinghua University have assembled a version that fits on a lab bench. It is designed to calibrate weights from 1 gram to 1 kilogram with E2-class accuracy — meaning the error does not exceed 0.27 parts per million. If you weigh a kilogram standard, the error would be less than a third of a milligram — about the weight of a grain of sand.
The key trick of the new design is how the coil is moved. During measurement, the coil must move at a constant speed, but in practice it accelerates and decelerates. The researchers proposed adding extra oscillations, odd harmonics, to the current that drives the coil. These smooth out the speed, much like an experienced driver smooths out the jerks of a car on a rough road. An even harmonic corrects for differences between upward and downward motion.
As a result, the coil's speed remains nearly constant: the deviation stays below 5% for 60% of the motion cycle. This makes measurements stable and precise. Such tabletop balances could appear in every calibration lab, making precise mass measurement as routine as measuring temperature.
The modern kilogram is no longer stored as a metal cylinder in Paris. Since 2019, it is defined by the Planck constant — a quantum quantity that can be measured in any lab using a Kibble balance.