Weighing up the future of the kilogram
Scientists have met at the Royal Society in London to discuss the future of the world's standard measurements.
The original kilogram, a chunk of platinum-iridium metal held in a vault in France, is the last metric unit still defined using a physical object.
Everything else, from seconds to metres, is now measured using fundamental scientific constants. As these are unchanging, the units can be measured to extraordinary precision anywhere in the world.
Terry Quinn, who organised the meeting, is former director of the International Bureau of Weights and Measures (BIPM) in France, where the kilogram mass is kept.
He explained to the BBC's Jason Palmer what the meeting hoped to accomplish, and how.