# Scientists just ditched the single lump of metal that defined the kilogram for 129 years for a better measure

- Over 50 scientists have voted in a change which will see the 129-year-old method of defining a kilogram scrapped.
- The new method, decided at the General Conference on Weights and Measures in Paris on Friday, will be a formula called the "Planck constant."
- A lump of platinum-iridium called "Le Grand K," which only left its protective case four times in the last century, was used as the previous definition.
- They made the change as the "Grand K" was losing mass: equivalent to the weight of a eyelash.

Scientists in Paris on Friday approved a change which will alter how a kilogram is defined, the first change to the system of measuring mass which has been in use since 1889.

Scientists from over 50 countries voted to change the definition to a scientific formula called the "Planck constant (*h*)," which works out a kilogram based on the amount of electricity needed to counteract its force.

The previous way scientists defined a kilogram was by the weight of a "platinum-iridium lump," called "Le Grand K," which is kept securely at a site just outside Paris, but will now be retired.

This lump was established as the singular measure for all kilograms in 1889 and has only left its protective case four times in the last 100 years, Sky reported.

But the lump, and the identical copies of it, experienced a minute amount deterioration according to Pallab Ghosh, the BBC's science correspondent.

In fact the changes were so small they were about 50 parts out of a billion, or the weight of an eyelash. By definition, whatever the lump weighed was a kilogram, so if a chunk fell off, then the mass of all the matter in the universe, if measured by kilograms, would change.

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Perdi Williams from the National Physical Laboratory said: "The new system is going to work a lot better. It is also a really exciting time, and I can't wait for it to happen," the BBC reported.

At the event, organised by the International Bureau of Weights and Measures (BIPM,) several other units of measurement have had their definitions changed: the ampere, the kelvin, and the mole.

The ampere will be defined by the elementary electrical charge (*e*,) the kelvin will be defined by the Boltzmann constant (*k*,) and the mole will be defined by the Avogadro constant.

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