The new kilogram prototype
How heavy is a kilogram? The answer to this question can be found in the form of a metal cylinder in a safe at the International Bureau of Weights and Measures in Sèvres near Paris. Up to now it has served as prototype against which everything else is measured. But this kilogram prototype appears to be unstable. There are indications that the mass of the kilogram prototype has changed. If it were destroyed or stolen the definition for a kilogram would be irretrievably lost.
That is the reason for the international efforts that have been going on for years to redefine the unit of measure and find a constant gauge. At the National Metrology Institute of Germany (Physikalisch-Technischen Bundesanstalt (PTB)) a solution is being sought for this in natural constants – in atoms that comprise a certain crystal. Pure silicon is key to the definition of a new, constant kilogram.
From an ultrapure silicon ingot the most perfect spheres possible are produced. These are measured precisely and the atoms counted. Based on the weight of a silicon sphere and the number of the atoms it contains researchers want to determine how many silicon atoms constitute a kilogram.
To align the spheres for the numerous measurements, markings are necessary that inflict the least damage possible. In its search for a suitable method the PTB in Braunschweig found what it was looking for at Micreon.
The result: perfect silicon spheres as constant new mass marked with a landmark in a loss-free process by ultrashort laser pulses.
The final new definition of the kilogram is planned for 2018.