Volts and amps are not what they used to be: Nor are several other common units of measurement, now that technology can link them to natural constants and provide portable standards of unprecedented accuracy

If engineers ever allowed themselves to have sacred objects, the standard kilogram would certainly be one. Made of a platinum-iridium alloy, it rests at the International Bureau of Weights and Measures (Bureau International des Poids et Mesures, or BIPM) in Sèvres, outside Paris. It is a uniquely treasured entity, kept inside a set of nested enclosures that protect it from contamination and mishandling. It is cleaned only with live steam from specially purified water.