It was a measurement equivalent to the weight of a liter of water that was fixed by the Government of France to 1795, while the country waged war against the European powers that wanted to squash the revolution. The ampere, Kelvin and mol also will be reviewed the British company Johnson Matthey, still in operation, polished with care a cylinder of 39 millimeters of high platinum fact so that its weight adjusted most possible to the one of the liter of water. The weight is conserved since then in the camera of the BIPM next to six copies. The copy and weights original, more of a century later, are mysteriously different. ” We do not know if they are gaining mass and others losing it nor of how much mass we are speaking exactamente” , Mills recognizes. The difference is so small that it will not cause fruit fights between and clients, but scientific debates in laboratories worldwide and problems for the students. ” The objective of the redefinition is not to make simplest the concept possible, but to determine a mass unit that is most stable possible over the years ” , it argues Stock.

Stability and precision the reason of the change is that it does not have nothing else stable to which the original kilo can be compared to determine its true weight. It is here where the Planck’s constant enters, an immutable value in the nature that will be used like reference to express the value of the kilo. In 1983, already another constant of the nature, the speed of the light was used, to redefine the meter. There are two forms to repesar the kilo with the maximum precision. One is to use a power balance. In a side the kilogram is placed and in another bovine one around which they circulate current electrical that reproduces the Planck’s constant, near, but no equal, to the weight of an electron. Of the five balances that there are in the world (the USA, Canada, Switzerland, France and the BIPM), only the American has contributed results until the moment.

The other form is to use the constant of Avogadro, that is the number of carbon atoms 12 that there is in 12 grams of that substance. As as much the one of Planck as the one of Avogadro they are constant of the nature, its estimations of the kilo would have to be identical, but they are not it. ” The discrepancy is of a part between ten million and the objective is that it is not superior to two or three parts each one hundred millones” , it summarizes Mills. It is not the unique problem. The redefinition needs additional confirmations other experiments that will not arrive until within two years, in the case of the power balance that uses Stock and other still more precise weights that still they are in construction. Nobel Hall asks in addition who weighs more retorts of the kilo, including which they use the companies, before the new value is adopted. Altogether, the redefinition process takes in march from 1976, according to Stock. Long before which it finishes, perhaps this year, the three guards of the kilo will return abrir the camera and will take the weight for new pesaje that could give new surprises.