The NIST "D.I.Y." Watt Balance is a classroom science project that teaches the principles behind the ongoing effort to redefine the world's basic unit of mass, using LEGO® bricks and some basic electronics.
The NIST "D.I.Y." Watt Balance, made with LEGO (R) bricks, is a science project that teaches the principles used in the international kilogram redefinition effort.
Video by Jennifer Lauren Lee/NIST PML
For more than a century, the kilogram (kg) – the fundamental mass unit in the International System of Units (SI) – has been defined as exactly equal to the mass of a small polished cylinder, cast in 1879 of platinum and iridium, which is kept in a triple-locked vault on the outskirts of Paris. This "one kilogram to rule them all" is known as the International Prototype of the Kilogram, or IPK.
But there are inherent problems with using physical artifacts as definitions, since these objects appear to experience slight changes in their mass over time. The current system also requires that nations conduct the time-consuming business of periodically sending their own official kilograms to France for comparison with the IPK.
For these and other reasons, the international community of measurement scientists is currently developing a new, more stable definition of the kilogram, one that is based on a constant of nature. (Read more about the worldwide effort to redefine the kilogram.)
A key piece of machinery used to make this redefinition possible is the watt balance. Watt balances work by making an indirect comparison between mechanical power and electrical power – that is, they measure the downward pull of a gravitational force on a mass by counteracting it with an upward electromagnetic force.
Their design makes watt balances capable of ultra-precise mass measurements: for example, one machine currently being built at the National Institute of Standards and Technology (NIST) in Maryland, USA, should be able to measure a mass to within three millionths of a percent. (Read more about watt balances.)
Not sure if building a homemade watt balance is right for you? Watch the video above.
Ready to build your NIST Do-It-Yourself Watt Balance? See this journal paper for instructions and a list of parts. (You'll be directed to a web page where you can download a PDF. Use the link in the top right corner of that page.)