For more than a century, all measurements of weight have been defined in relation to a lump of metal sitting in Paris. The "international prototype" kilogram has been at the heart of trade and scientific experiment since 1889, but now experts want to get rid of it.
Today, scientists will meet at the Royal Society in London to discuss how to bring the kilogram into the 21st century, by defining this basic unit of measurement in terms of the fundamental constants of nature, rather than a physical object.
"The kilogram is still defined as the mass of a piece of platinum which, when I was director of the International Bureau of Weights and Measures, I had in a safe in my lab," said Terry Quinn, an organiser of today's meeting. "It's a cylinder of platinum-iridium about 39mm high, 39mm in diameter, cast by Johnson Matthey in Hatton Garden in 1879, delivered to the International Committee on Weights and Measures in Sevres shortly afterwards, polished and adjusted to be made equal in mass to the mass of the old French kilogram of the archives which dates from the time of the French Revolution. Then, in 1889, it was adopted by the first general conference for weights and measures as the international prototype.
One problem with using a lump of metal to define such a basic quantity as the kilogram is that it is liable to change over time. Measurements over the past century have shown that the international prototype has lost around 50 micrograms, around the weight of a grain of sand.