The kilogram, along with several other standard units, is being redefined by the International Bureau of Weights and Measures.
How much does a kilogram weigh? It’s a simple question with a complicated answer – and next May, that answer is changing for the first time in over a century. As it stands, a kilogram is defined by a lump of metal kept in a safe in Paris. Officially called the International Prototype of the Kilogram (IPK), but more affectionately referred to as Le Grand K, it’s a cylinder made from a platinum-iridium alloy that has defined the kilogram since 1889, when it was officially sanctioned and put into storage at the International Bureau of Weights and Measures (BIPM) in France. There are also several copies of it around the world – the UK’s specimen is held by the National Physical Laboratory (NPL) in Teddington.
Before the metric system was created, weights and measures varied from nation to nation, and even within countries. But after the French Revolution, the movement to create a universal standard started gathering pace. Eventually the first physical standards were created, Le Grand K among them. Over time, scientists have replaced these standards with more robust definitions – with the kilogram the last to go.
For decades, metrologists (scientists who specialise in measures) have known that Le Grand K’s weight changes due to atmospheric conditions. Being a physical object means the IPK is subject to its environment: pollution in the air can bind to the metal, adding weight, while wear caused by handling can decrease its mass. Every 40 years, the IRK and its copies are compared. Those measurements show the weight of the copies increasing by tens of micrograms, relative to the original which remains, by definition, 1kg.
In November, scientists voted to scrap the existing definition of a kilogram in favour of a new one based instead on a fundamental constant of nature known as the Planck constant. The new definition applies from 20 May 2019. “Scientists, and more specifically metrologists, should be pleased they now have a perfectly stable definition for the SI unit of mass,” says Dr Stuart Davidson, head of mass metrology at NPL. “This will impact pharmaceutical research and production, microfabrication and robotics, and in the longer term it could be used in areas such as personalised medicine.”
The new definition will work using a Kibble balance, which uses a currentcarrying wire in a magnetic field to balance the mechanical energy exerted by an object with mass. Scientists can then calculate the weight of the object by moving the coil and measuring the speed with a laser, and using universal constants such as the speed of light and Planck’s constant.
It’s not just the kilogram that’s getting a new definition. Our measures of temperature (Kelvin), electric current (ampere) and amount of substance (mole) are all getting new definitions, also based on fundamental constants. The other three base units defined by the International System of Units – our measures of time (second), length (metre), and luminosity (candela) – are already based on fundamental physical constants, so will not change.
WHAT IS THE PLANCK CONSTANT?
The Planck constant, 6.62607015 × 10-34 kg m2 s-2, alongside other physical constants like the speed of light, is believed to be a fundamental property of the Universe that
does not vary over time. The constant is used to calculate the amount of energy in a photon, by multiplying it by that photon’s wavelength. It’s named after Max Planck, a German theoretical physicist who won the Nobel Prize in Physics in 1918 for figuring out that energy comes in packets, or quanta, kick-starting the field of quantum mechanics. It’s taken years to measure it to the level of accuracy needed but that point was finally reached in 2017, making the kilogram’s new definition possible.
Words: Kelly Oakes
Kelly Oakes is a freelance science journalist with a degree in physics.