A massive problem

The need for such a machine has arisen because comparison weighings of the prototype one kilogram mass of platinum iridium at the Bureau International des Poids and Mesures near Paris and other standard kilogram masses show that their weights are diverging. The problem has been known since 1946 and, according to Prof Ian Mills, president of the Consultative Committee for Units at the BIPM, it has now reached a point where the uncertainty in how much mass there is in a kilogram is about one part in 10⁷. 0.1mg in 1kg may not sound a lot, but is more than enough to put a space probe off course and contrasts badly with quantities such as the metre, which is defined to 1 part in 10¹¹, and the second, to one part in 10¹⁵, The second and metre are defined in terms of fundamental physical measurements and their precision is now considered essential for the proper operation of telecommunications and GPS systems. The solution devised by Dr Bryan Kibble at NPL is to make use of what he describes as a watt balance. Originally invented to define the ampere in terms of the metre, kilogram and second, it can be used in reverse to define the kilogram in terms of Planck's constant. Dr Kamal Hossein, director or research and international cooperation at NPL noted: "Having demonstrated that it works, it requires more work for a longer time. We now have other priorities. Our work on the SI system does not make any money. We did offer it to the BIPM but they are building their own, however, Canada was willing to take it. We have taken some money but it was not done to make a profit".