Kilogram: The Kibble Balance

On November 16th, 2018, metrology experts from more than 60 countries gathered in the 26th General Conference on Weights and Measures (CGPM) at the Palais des Congrès in Versailles, France and witnessed the important vote on the redefinition of four international (SI) units: the kilogram, the ampere, the Kelvin, and the mole, to the fundamental constants in nature. These changes have also come into force on May 20th, 2019.

The highlight of these changes is the redefinition of the kilogram by the Planck constant, because finally, human beings can abandon using a specific manmade artefact to define the units of mass. Therefore, the International prototype of kilogram (IPK), which is a 1 kg Weight made of platinum-iridium alloy stored in the headquarter of the BIPM in Sèvres on the outskirts of Paris, as well as its replica, can now be retired after their 130 years of faithful service.

One way to realize a new IPK is through utilizing the principles of electromagnetism, and one such device is the Kibble Balance (a.k.a. the Watt Balance). In this device, weighting a mass is achieved by precise measurement of the emf (electromotive force) voltage to calibrate the balance and then the electrical current required to generate an electromagnetic force against the gravity. In this activity, we will present a LEGO version of the Kibble Balance, which can measure a gram-level mass, with a theoretical precision of better than 1%.