A LEGO Model of the Kibble Watt Balance for Physics Education and Outreach

In 2019, the International System of Units unit of mass, the kilogram, was redefined based on the fixed value of the fundamental Planck’s constant, therefore eliminating the need for the International Prototype of Kilogram, the platinum-iridium cylinder that was forged in 1879. The apparatus that allows to realize the kilogram based on the Planck’s constant has been constructed at the National Institute of Standards and Technology (NIST). It is based on the idea of Bryan Kibble to balance the weight of the object by the electromagnetic force generated by the current-carrying coil immersed in a magnetic field, therefore the name, Kibble Watt balance. The apparatus is housed in a dedicated room under clean-room conditions and can measure a kilogram within a few parts in 10⁸. In 2015, a simple LEGO model of the Kibble Balance was constructed by NIST scientists [L. S. Chao et all, American Journal of Physics 83, 913 (2015); doi: 10.1119/1.4929898] and has been replicated several times. Since 2015, the design has been changed because of the efficiency and availability of the components.

Our chapter of the Society of Physics Students decided to construct a LEGO Watt Balance model based on the new design for our 2022 Research Project which has been funded by the SPS National research grant. We really enjoyed working on this project, and believe this is a very good undergraduate project, even for a small physics department. We also plan to use the model for recruitment and outreach. In this presentation, we will share the obstacles we had to overcome and the lessons we have learned while making this inexpensive table-top model capable of measuring gram-level masses with 1% uncertainty.