What is a kilogram?

A question that is too easy for her . to sell meat on the market, but too difficult for home . physics!

It is not difficult because of the complicated concept, but it is difficult because of its both specific and ambiguous definition. The definition of kilograms is very arbitrary: "Kilogram is a unit of mass. It is equal to the mass of international prototypes of kilograms."

1kg form in France (Photo: physics.nist.gov)

The kilogram prototype at the International Institute of Metrology is a small cylinder of platinum and iritium, which has been stored as a gold store since 1889 in Paris, France. At that time, this alloy was chosen because of its sustainability, expressed in its resistance to corrosion, acid, and because it has a high density, it has less force when weighing. most impacted.

It was placed under 3 glass bells, protected from dust attached to a safe with 3 locks, held by 3 leaders. Every year they meet to " look " at the kilogram prototype without touching it.

Occasionally (once every 50 years), the prototypes are taken to compare with the " model " (subsample) after being " bleached " according to a sophisticated procedure, using impregnated camel skin in a mixture. alcohol and ether applied.

The kilogram pattern is difficult to reproduce and is difficult to popularize, because it is impossible to define what a "kilogram" of 1 kilogram is. Can only say what it is, where to put it. For example, the definition for 1 meter is: 1 meter is the length that light can travel in a vacuum, for a period of 1 part 299792458 seconds; seconds are defined from the 133 atom atoms. All well-equipped laboratories can find the necessary, basic units by relying on the definition, except for the kilogram definition.

A very important issue is that kilograms currently tend to lose mass with time due to wear . 10-3 with conventional measurements is an unspecified error, but with the correct determination of a physical unit, that error is an indispensable problem.

Scientists are very headache about this issue. A new approach has been proposed, which is the Watt balance using a comparison of electromagnetic forces with mechanics. The project aims to link the kilogram with an immutable quantity: Planck's constant, the fundamental constant of quantum theory, represents the smallest quantum. The French National Institute of Metrology also has this scale, the latest one located at the national laboratory in Trappes.

Currently Watt scales are being evaluated as effective ways to solve problems.

The kilogram pattern can still rest its throne in three bells in Sèvres. But maybe another 15, 20 years people will put it in a museum of modern science.