Every measurement is a comparison between a quantity being measured and a standard 'unit' amount of that quantity. Through a chain of comparison measurements, every measurement of mass is a comparison to the mass of a particular piece of metal held in a vault on the outskirts of Paris. Since 1889 this piece of metal - International Prototype of the Kilogram – has defined what a kilogram is. But scientists suspect that the mass of the lump is slowly changing compared to true constants found in nature. These 'natural' constants have been impossible to exploit in a practical way—until now. Scientists have at last worked out a new and better way to define what we mean by a kilogram.

See Dr Terry Quinn CBE FRS explain the science behind his exhibit in the video above,

How it works

At the moment mass measurements are made by comparing the gravitational force on a mass, with the gravitational force on the International Prototype of the Kilogram. In future, scientists propose measurements of mass can instead be made in terms of the electromagnetic force required to balance the gravitational force on an object. The strength of this electromagnetic force can be related to fundamental constants of nature: the Planck constant, the speed of light and the frequency of the caesium atomic clock.


In order to make the change of definition seamless, we need to compare the gravitational and electromagnetic forces and make sure their ratio depends only on mass and nothing else. To check that, scientists around the world have built special balances – known as watt-balances – on which we can apply known electromagnetic forces and compare them with known gravitational forces. As the home-made version on the stand demonstrates, the basic idea of a watt balance is not complicated.