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NIST donates watt balance made of LEGO® bricks to the BIPM

The project to measure the Planck constant, h, to unprecedented uncertainty (about 0.000 002 %) is a prerequisite to redefining the kilogram. The new definition is on track to be approved in 2018. One way to achieve this goal is by means of an electro-mechanical device known as a watt balance.

In order to acquaint a wider community with the principles of a watt balance and to have some fun in the process, NIST researchers have developed a simplified version of the laboratory apparatus made almost entirely of LEGO bricks. Thanks to the generosity of NIST, BIPM has received one of these devices to show our many visitors the basic features of the much more elaborate laboratory device. The demonstration balance operates over a range of 1 g to 20 g and can determine the value of h to better than 1 %. The "technology transfer" between NIST and the BIPM took place during the week of the CCM meeting in February, and BIPM staff have since demonstrated the watt balance to attendees of the CCEM meeting in March and the CCQM meeting in April.

See Facebook.com/LEGOwattbalance for more information.

Technical explanations and a full parts list are available on arXiv.

In the spirit of the NIST D.I.Y. watt balance, the BIPM has developed a second demonstration of the measurement of h. This is based on a 20 g cube of polycrystalline aluminium which shows how the new definition of the kilogram will be realized from the physical properties of a near-perfect sphere made of single-crystal silicon [1]. The aluminium cube was manufactured at the BIPM and can itself be used to determine h to better than 1 %. The cube can even be weighed on the D.I.Y. watt balance to show the two methods are comparable and complementary.


  1. Davis R.S., What is a kilogram in the Revised International System of Units (SI)?, J. Chem. Ed., 2015.
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