The Reference Kilogram

By Deane Barker on June 20, 2012

The Search for a More Perfect Kilogram: How do we know what something weighs?  In particular, a base unit like the kilogram?  Well, it turns out, a kilogram is only a kilogram in comparison to several objects which essentially “claim” to be a kilogram.

The official US kilogram — the physical prototype against which all weights in the United States are calibrated — cannot be touched by human hands except in rare circumstances. Sealed beneath a bell jar and locked behind three heavy doors in a laboratory 60 feet under the headquarters of the National Institute of Standards and Technology 20 miles outside Washington, DC, the shiny metal cylinder is, in many ways, better protected than the president.

This isn’t the only one.  There’s one in France they called “Le Grand K” which is even crazier.

Aside from a yearly ceremonial peek inside its vault, which can be unlocked only with three keys held by three different officials, the prototype goes unmolested for decades. Yet every 40 years or so, protocol requires that it be washed with alcohol, dried with a chamois cloth, given a steam bath, allowed to air dry, and then weighed against the freshly scrubbed national standards, all transported to France

The problem is that the weight of some of these reference objects is slowly changing.  Since many other measures are based on the kilogram, if it changes, there would be a ripple effect that would affect the measurement of all sorts of different things. So, there’s an effort on to redefine the kilogram to be some natural constant, rather that just reference a thing that might degrade in some way.