Tagged: Planck constant

How much does a kilogram weigh?

IPK

The International Prototype Kilogram is under all of those bell jars

This is a picture of the international prototype kilogram, or IPK. It is exactly, and always, a kilogram. It’s a big lump of metal which the world has agreed is how much a kilogram weighs. It was made in 1879, and it is still beautifully, perfectly, and exactly the same as it was then.

Er, more or less. And that is the problem.