The Kilogram

Did you know that the kilogram is the only SI base unit that is still directly defined by an object - the International Protoype Kilogram?  I was thinking about the kilogram this evening as I was grading the 3-2-1 reflection assignment.

The image below shows the current International Prototype Kilogram
(under two glass jars for protection)

http://www.bipm.org/utils/common/img/mass/prototype.jpg

 According to Wikipedia...

..."three other base units (Cd, A, mol) and 17 derived units (N, Pa, J, W, C, V, F, Ω, S, Wb, T, H, kat, Gy, Sv, lm, lx) in the SI system are defined relative to the kilogram, so its stability is important. Only 8 other units do not require the kilogram in their definition: temperature (K, °C), time and frequency (s, Hz, Bq), length (m), and angle (rad, sr)."

 A silicon sphere is a potential replacement for the Kilogram standard

http://www.nist.gov/pml/si-redef/images/SiliconSphere-Closeup-800_1.png

According to a document on the NIST webpage...

..."the international metrology community has chosen to answer those questions by defining the kilogram in terms of either a silicon sphere or an invariant of nature called the Planck constant for its discoverer, Max Planck, one of the pioneers of quantum science."

Maybe one of my students will  have a job at the NIST some day and help to redefine one of the basic quantities we use for measurement in science!