The kilogram based on the Planck constant definition of the kilogram is based on a fundamental constant of quantum physics, Planck’s constant, have chosen metrology experts convened by the International Bureau of weights and measures. So has communicated in London physicist Michael Stock, the Bureau, on the occasion of a meeting convened by the Royal Society to assess progress in this topic: consensus has been reached so that in the near future will redefine the kilogram, based on a fixed value of the Planck constant. Until recently there was another possibility, explored the Avogadro project, which was that the kilogram is based on Atomic masses. However, the results of this project, which has just been published, are complementary experiments around the Planck constant to redefine the unit. According to Stock, scientists have spent years performing with so-called Watt balance experiments that establish a relationship between the mass and the above constant by comparing power measurements electrical and mechanical, but the new definition may not enter into force until these measures give identical results for what more Watt, a very complicated instrument scales are necessary.
The kilogram is the only unit of the international system of units that is still defined on the basis of a physical object, a prototype of Platinum and Iridium remains saved in the Bureau in Paris headquarters. The rest of the units (meter, second, ampere, kelvin, Mole and candela) are based on physical quantities (the metro is defined on the speed of light), although the kelvin is in a phase of redefining also. The measures of the prototype of the kilogram over the last century have indicated that its mass has changed ligerisimamente, and is now about 50 micrograms (like a small grain of sand) bottom when it was built in 1879. The ultimate goal of the work is that all units of the international system are stable and universal. Original author and source of the article.