Watt Value of Plank's Constant is a Kilogram? (August 25, 2009)

The kilogram is the only unit of measure still defined by a single object — a lump of platinum-iridium about the size of a salt shaker held in a vault near Paris. Over time, as atoms accrete or fall off this particular kilogram, its mass changes.

While the meter was originally defined at the distance between two scratches on a platinum/iridium bar, in 1983 it was redefined as the distance light travels in a vacuum over 1/299,792,458 of a second.

To define the kilogram has proved to be a far more daunting challenge, i.e. to redefine it with respect to

Plank's constant (6.626 068 96(33) × 10⁻²⁷ ergs)

To listen to Geoff Brumfiel discuss the difficulties and the use of the Watt balance to nail the definition Click Here (5½ minutes).