Last Friday, the 26th General Conference on Weights and Measures voted to replace “Le Grand K”, the 129 year old physical prototype of the kilogram, with a new measure based on physical constants. The new definition, which comes in to force next spring, is now fixed by defining the Plank constant as exactly 6.626 070 15 x 10^-34 Joule seconds. 

The kilogram was the last SI unit based on a physical prototype (which means that until next year, Le Grand K is, by definition, exactly one kilogram). Other prototypes, such as the physical meter, have been long since retired. This redefinition is a triumph of high-precision experimental physics. The problem with physical prototypes for units is that they can change. Le Grand K, for example, has been losing mass, to the tune of about 50 micrograms (5 parts in 100 million), possibly because the identical cylinders it is compared to have gotten dirty from being handled more often.

Farewell Le Grand K! Thank you for your service!