Much to the consternation of scientists, the cylindrical platinum-iridium artifacts that represent the kilogram (see image above) have been gradually packing on extra weight due to surface contamination. Since that unit of measure is the last to be based on an artifact and not a physical constant of nature -- for instance, a meter is the distance light travels in a vacuum during 1/299,792,458 of a second -- it means that scientists no longer know exactly how much a kilogram is. That makes experiments requiring extreme precision more difficult, so researchers from Mettler Toledo, CERN and the EPFL have been working for the last 15 years on a so-called Watt balance, which works on the principle of electromagnetic force restoration. The team managed to created a "load cell" that's accurate to a 0.3 µg resolution for a 2kg weight, well below the desired level of 1 µg -- meaning the goal of replacing a hunk of metal from 1878 with something more, ahem, solid is within reach by the 2015 target date.
Source: Mettler Toledo