What is a Watt Balance?

These are “exciting” times for meteorologist. In 2018, it is expected that all of the fundamental SI units will be defined, including the kilogram. Right now, a kilogram artifact sits in France. It is the kilogram which is agreed that will be the definition of the kilogram, period. So if it shifts by a microgram, the reference standards in the national laboratories will shift by a microgram. Wouldn’t it be better to have a definition of the kilogram based on fundamental constants of the universe rather than an artifact? That is what is going on right now.

First, take a look at this short video that describes a watt balance is and how it works. This will help you understand the video that follows.

watt balance diagram

And of course, there has to be some vacuum technology involved!

Watt Balance.PNG chamber.PNG

The NIST-4 watt balance has measured Planck’s constant to within 34 parts per billion, demonstrating that the high-tech scale is accurate enough to assist with 2018’s planned redefinition of the kilogram. The watt balance is shown sitting in the bottom half of its vacuum chamber (left) and with the vacuum dome installed (right).

Credit: J. L. Lee / NIST

YouTuber Veritasium takes us into the National Institute of Standards and Technology to see one of the American standards.


You can learn much more at the NIST Physical Measurement Laboratory site.
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NIST CODATA 2010 wallet card 2.jpg

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