A kilogram is a kilogram. Well, may be not…

According to the article New look for the kilogram, published on PhysicsWeb, researchers from University of Reading and NIST have proposed new methods to define kilogram, based on fundamental constants. The kilogram, one of the seven International System of Units (SI) base units (the only which is still defined by a material artefact) is defined as the mass of the international prototype of the kilogram kept at the Bureau International des Poids et Mesures (BIPM). This prototype is a cylinder made of an alloy for which the mass fraction of platinum is 90 % and the mass fraction of iridium is 10 %. The problem is the mass of the international prototype increases by approximately 1 part in 109 per year, due to the inevitable accumulation of contaminants on its surface. In this context, scientists have proposed two ways to redefine the unit of mass. The first is based on the Planck constant and requires a 1-kilogram mass to be supported against Earth’s gravity using a precisely measured magnetic force. The second technique is based on the Avogadro constant, and involves counting a certain number of atoms of a specific atomic mass. Nevertheless, none of these definitions will be adopted before 2007. Until then we can continue to trust our balances measurements…