France’s 138-year-old official kilogram weight checked

The high-precision weighing experiments needed to define the amount are continuing worldwide ahead of the change, after the General Conference of Weights and Measures (GCWM) voted in 2014 to transfer the official definition of the kilogram to a new standard measure, as reported in French newspaper Le Monde.

The new definition will be finally drafted - pending experiments - at the next GCWM, in 2018.

Previously, the weight was officially defined by “the Big K”, a 139-year-old, physical cylindrical measuring block made in France.

Created in an alloy of platinum and iridium, it is stored in the International Bureau of Weights and Measures in Sèvres, south of Paris, with six copies of itself.

However, for several years, it has been known that this official “Big K” appears to have been “losing weight”, compared to its “witness” comparisons held around the world - although scientists disagree whether this is because its metals are losing mass, or because the “witnesses” are gaining weight.

To combat this, the official kilogram weight is now likely to be defined by “Planck’s constant” weight, which was first defined a little later, in 1900. This is a value from quantum physics, which - to use its simplest definition - describes how energy is perceived.

Very difficult to measure properly, scientists are using a “kibble balance” to define it, which uses electromagnetism and a coiled wire to calculate the constant. Experiments are still continuing to ensure that the calculations are accurate enough to be accepted by the GCWM. It is this measure that is likely to be used from 2018.

The official weight of a kilogram matters, because other weights and measurements are defined by it, especially the joule (the amount of energy needed to move one kilogram by one metre), and the candela (the brightness of light, measured in joules per second).

Even tiny discrepancies could cause huge problems with technology, especially as computer speeds get faster and more precise.

It is not the first time than an official weight has been questioned and changed; the official metre used to be defined by a rod stored next to the Big K, but scientists decided to change it to the “distance light travels in a vacuum during 1/299,792,458 of a second”.

This is significant, because unlike a physical piece of metal, a precise mathematical calculation is extremely unlikely to change.