The Search for a More Perfect Kilogram
No one can say for sure why the prototype and its brethren are drifting apart. One rather obvious possibility, suggested by Taylor, is that the national prototypes and even the témoins have been used more often than Le Grand K, which has been handled only three times since 1889. The handling could subtly contaminate the surface. A more exotic theory posits that slight variations in Matthey’s alloy lead to different rates of outgassing, the technical term for the gradual escape of gases trapped in the metal. Whatever the explanation, the divergence is problematic, and not only for theoretical reasons. In fields ranging from particle physics to global commerce, the erratic behavior of the master kilogram shows that a system of measurement based on a physical artifact can’t be trusted. “This is simply not a satisfactory situation,” Quinn says. “You have an object made with the technology of the 19th century upon which a very large proportion of modern measurements are based — not just mass, but electrical measurements and measurements of force and heat and light.” The metric energy unit known as the joule, for example, is defined in terms of the work needed to move a 1-kilogram mass a given distance over a given time period. And the luminosity of light, or candela, is measured in terms of power, designated in watts, or joules per second. In other words, if the kilogram is unreliable, the joule and the candela become unreliable as well. Nobody at the grocery store is fretting over whether a kilo of bananas is a speck of dust lighter or heavier than in their great-grandparents’ era, but the change could eventually matter enormously to engineers optimizing computers and fiber-optic networks.