You might be forgiven for thinking that what you're seeing here is a very well-constructed toy. But it's also a fully operational watt balance, the tool that's used to set the standard measurement of a kilogram.
Scientists at the National Institute of Standards and Technology designed and built the fully-functional, though toy-based, watt balance, and then detailed their process in a working paper that they posted on ArXiv (it's also been submitted to the American Journal of Physics).
The LEGO version of the watt balance is not the very best version of the equipment out there. The researchers note in the paper that there are some other existing watt balances out there in the world that have calibrated their relative uncertainty to within less than fraction of 1%. So why attempt to construct an equivalent LEGO version? For a couple reasons. For starters, even though the LEGO version is not the most accurate one, it's still a pretty darn good one, with a relative uncertainty of 1%.
Also, though, the standards of measurement and how they're derived are due to undergo a huge update in 2018. And, note the researchers, as a way of explaining the new concept behind how a kilogram is derived, the LEGO watt balance is pretty unbeatable:
In the new SI, the kilogram will be defined via fixing the Planck constant, a way that is not apparent to most people. The LEGO watt balance makes it easy to demonstrates how electrical power relates to mechanical power. One can show how it is possible to generate a mechanical force, whose value is precisely given by electrical measurements. Unfortunately, it still requires some abstraction to explain how electrical power is related to the Planck constant via the Josephson effect and the quantum Hall effect. But once that bridge is crossed, the relationship between mass and h can be made clear. From there, it is fascinating to ponder the implications of mass redefinition: the Planck constant, a natural constant found in quantum mechanics, can be directly used to define mass on a macroscopic scale! At any scale for that mat- ter. With this LEGO balance, one can determine the absolute mass of small objects without needing any comparison or traceability to reference mass standards. It is also interesting to consider how an apparatus assembled from plastic bricks can measure h in a classroom or living room setting with an uncertainty of only 1 %.
So, basically, it's for science, for fun, and for us. If you're interested in building your own, the researchers have also included a full list of which LEGO parts they used to put together their machine, along with a cost breakdown.
Altogether it came to about $600, not bad for a toy that also doubles neatly as a lab tool.
Image: Xiang Zhang