This video introduces a do-it-yourself “tabletop” watt balance, built by researchers at the National Institute of Standards and Technology, who also built the NIST 4 watt balance.
Here the researchers themselves explain the principles and design highlights of both watt balances. The tabletop watt balance can measure mass to about 1% precision and costs about $400. It is made from standard LEGO pieces and a few electronics. The NIST 4 watt balance is about a million times more precise and not surprisingly more expensive, with many custom-made parts out of special metals.
Both watt balances work on the same principle: equating mechanical power to electromagnetic power. An object goes on one side and is balanced by electromagnetic force on the other side.
While a fun and useful DIY project, the watt balance has an even larger scope: the nature of this experiment can determine one of the fundamental constants of physics: Planck constant h. h can be determined with a known mass (m), gravitational acceleration (g) and velocity (v) of the moving mass. By determining a value for h, this type of experiment is crucial to the redefinition of the kilogram in 2018.
For a shopping list and more details about building your own watt balance, see “A LEGO Watt Balance: An apparatus to demonstrate the definition of mass based on the new SI” L.S. Chao, S. Schlamminger, X. Zhang, D.B. Newell, J.R. Pratt, F. Seifert, G. Sineriz, A. Cao, D. Haddad. arxiv.org/abs/1412.1699
Part 2 is available here: vimeo.com/167492608
Credits for video “This is not a toy: A tabletop watt balance” (Part 1 of 2)
NIST Researchers of the Fundamental Electrical Measurements Group: Leon Chao, Darine Haddad, Jon R. Pratt, Stephan Schlamminger
Filmmaker: Amy Young
Science Advisor: Michael Trott
This is a "pre-take" for the documentary film "The State of the Unit: The Kilogram", also by Amy Young. For details about the documentary, see stateoftheunit.com