New definition of kilogram finds success

Nature : For 126 years, the kilogram has been officially defined by a cylinder of platinum and indium in a high-security vault outside Paris. A physical object is perpetually at risk for changing due to the loss or addition of particles, so an ongoing effort to redefine all the standard units has included redefining the kilogram. In 2011 the International Committee for Weights and Measures (CIPM) agreed to base the kilogram on Planck’s constant but required that three independent measurements be made, with two of them using different techniques. The two groups using different techniques have now provided their measurements and come to an agreement. The first measurement came from the international Avogadro Project, which counted the atoms in two silicon-28 spheres with the same mass as the standard kilogram. From that they calculated a precise value for Avogadro’s constant which could be converted into a value based on Planck’s constant. In 2012 Canada’s Measurement Science and Standards laboratory in Ottawa used a watt balance, which weighs a test mass against an electromagnetic force, and the measurement of the force was then converted to a value based on Planck’s constant. The US National Institute of Standards and Technology (NIST) also used a watt balance but was unsuccessful in matching the previous measurements until August 2015. The cause for the differences in earlier measurements is still unknown.