Measurement

# The Standard For The Kilogram May Be Losing Weight

The iridium alloy cylinder known as the International Prototype Kilogram, or Le Grand K, was forged in 1879 to set the standard for the weight of a kilogram. It sits beneath three vacuum-sealed bell jars in a vault outside Paris, coming out once every 40 years for a careful cleaning and a precise weigh-in to compare it with its 80 official replicas, which are used by countries throughout the world. These replicas are what scientists use to verify whether something is exactly one kilogram. But at its last weigh-in, something was off: its weight had drifted ever so slightly below that of its replicas by 50 micrograms, or roughly the weight of a grain of sand. Scientists don't know whether Le Grand K is losing weight, perhaps through cleaning or air gradually escaping the metal, or if it's the replicas that are gaining mass because of their less stringent handling requirements (even contamination from the open air might be enough to cause the difference). What we do know is that this difference means drastic consequences for the extreme precision of science. In truth, any tangible object is at risk of this kind of change, which explains why almost all of the seven standards of measurement established in 1791 -- all but Le Grand K -- have been abandoned, most in favor of universal constants. The meter, for instance, is now defined by how far light travels in a vacuum in a certain fraction of a second. In 2011, the General Conference on Weights and Measures decided to do the same with the kilogram. How they'll redefine it is anyone's guess, but the deadline for the decision is set for 2018. We've collected some awesome videos on this topic. Watch them now to learn more.

– SciShow

### Key Facts In This Video

1. In 1899, the International Bureau of Weights and Measures created an iridium cylinder as the worldwide standard for the kilogram. 00:32

2. 30 years ago, scientists realized that the masses of the original kilogram and its copies are drifting apart. Nobody knows why. 01:03

3. The BIPM has already redefined other measures using universal constants. A meter, for example, is the distance light travels in 1/299,792,458 of a second. 01:39

Curiosity Staff