Machined from a chunk of a virtually indestructible platinum-iridium alloy, the international prototype kilogram (IPK) was built to last for an eternity. And yet, being the last remaining, physical artifact in a club of fundamental SI units, the current definition of the kilogram has worn out. Most certainly the watt-balance will take its place, and redefine the kilogram with a true, physical phenomenon. [Grady] just built his own watt-balance from scratch, and he provides you with a decent portion of scientific background on the matter.
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