Watt-balance method accurately defines the kilogram

Researchers at the National Institute of Standards and Technology have spent years conducting experiments to find a reliable definition based in nature to replace the current international standard for the kilogram, a century-old cylinder of platinum-indium alloy about the size of a plum. Now, they have achieved that goal, using a watt-balance method they first tried in 1998. This time, they received the same results as with earlier experiments but with better accuracy, thanks to improved hardware.

The watt balance is a two-story-high apparatus that redefines mass in terms of fundamental physics and quantum standards. It measures the force necessary to balance a 1-kg-mass artifact against the pull of the Earth's gravity, as well as two electrical values. The watt balance is one of two leading approaches for redefining the kilogram. The other approach counts how many atoms of a specific atomic mass equal the mass of 1 kg. The latest NIST measurements have an uncertainty of 0.052 ppm compared with 0.087 ppm in the 1998 experiments. These measurements are more precise than any previous results by any research group using either approach.

For additional background on efforts to redefine the kilogram in terms of natural properties, see www.nist.gov/public_affairs/newsfromnist_redef_kilogram.htm.

National Institute of Standards and Technology, www.nist.gov.