IN A VAULT on the outskirts of Paris there is a small cylinder made of platinum and iridium that has the unique property that its mass can neither increase nor decrease. This property is not the result of new physics, however, because the object is over 100 years old. Rather, the unique properties of this cylinder arise because it is the international prototype of the kilogram. And the definition of the kilogram in the SI system of units states that “the kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram”.
This means that the cylinder always has a mass of 1 kg, even if it gets “heavier” as a result of accreting dirt from the atmosphere, or “lighter” as a result of cleaning. One wide-reaching consequence of defining the kilogram in terms of a material object is that if the mass of this object changes, the values of all the other masses and mass-related quantities in the SI system will change as well. This is obviously not a good state of affairs.
The fact that the existing definition of the kilogram has lasted so long, and caused so few problems, is a tribute to the physicists and metallurgists who contributed to the birth of the SI system of units, and to those who now maintain it. The prototype kilogram is kept by the Bureau International des Poids et Mesures (BIPM) in Sèvres, which takes great care with its storage and use.
The kilogram is occasionally taken out of its vault and used to calibrate the primary mass standards of each of the signatories of a diplomatic treaty known as the Metre Convention. Most countries in the industrialized world have signed the convention, and each signatory is entitled to a copy of the kilogram mass. The UK, for example, has copy number 18, which was one of the original copies fabricated in about 1894 - just five years after the prototype was made.
In each country a system is in place to ensure that all mass measurements can be traced back to the appropriate copy of the kilogram. At the National Physical Laboratory (NPL) in the UK we measure the mass standards of a small number of calibration laboratories, and these laboratories are then responsible for disseminating the results either to end users or to other calibration laboratories. This creates an unbroken chain of measurements linking the weighing scales in shops and the precision balances in universities and industry back to the national prototype, and therefore ultimately to the metal cylinder in the vault at Sèvres.
In the May issue of Physics World Ian Robinson at the National Physical Laboratory in the UK describes the search for a fundamental measure of mass in more detail.