Scientists will this week vote on whether to change the official definition of a kilogram.
Friday’s poll of scientists from nearly 60 countries will have profound implications for everything from pharmaceuticals to jet fuel.
All the world’s scales – including those that weigh in pounds and ounces – are currently calibrated against a small cylinder of titanium alloy which has been held in a triple locked Parisian vault since 1889.
“Le Grand K” has only been removed from its case four times in the last century.
Fluctuating
The problem, however, is that its weight appears to have changed ever so slightly over the years.
Even inside its protective case the object gets dusty and dirty – meaning it grows heavier. Meanwhile, giving it a wash will make it lighter.
Scientists estimate that it has lost around 20 billionths of a gram – approximately the weight of an eyelash – over the years. In a world that requires ever greater accuracy, that is a big deal.
As a result, scientists are calling for a new method of measurement.
"Le Grand K" - International Bureau of Weights and Measurements
Planck’s Constant
At a special conference in Versaille next Friday, experts from 57 countries will vote on whether to base the definition on an electric kilogram.
The new definition is based on Planck’s Constant – a number deeply rooted in the quantum world which relates the energy of a photon to the frequency of its waves.
It will allow scientists around the world to calibrate scales with unprecedented accuracy.
Industry
Stuart Davidson, a metrologist, at London’s National Physical Laboratory said an accurate agreed standard is essential for a range of industries.
"Once you get up to a few tens of tonnes - things like filling an aircraft with fuel - everything needs to be traceable back to a standard," he said.
"The same is true when you get down to very small masses like a milligram - for example the active ingredients of pharmaceuticals.
"You like to know you are getting the right dose of drugs when you are given a prescription."
Another NPL scientist Ian Robinson said the new definition will liberate the kilogram from its physical and geographical ties.
"You are not reliant on any one object anymore," he said.
"Effectively our mass scale is spread out and everyone can contribute. I see it as egalitarian - a form of democracy for mass."
Base measurement
The kilogram is just the latest standard unit of measurement to be updated as scientists work to redefine all base measurement units in terms of fundamental constants of nature.
In 1983, the metre was redefined from “one ten-millionth the distance from the Equator to the North Pole” to the “distance travelled by light in a complete vacuum in a tiny fraction of a second.
In 1967 meanwhile, the standard measurement of a second was changed from a fraction of the time it takes for the Earth to complete one rotation and is now defined by the vibrations in a caesium atom.
If the Friday’s vote passes – as it is expected to – a kilogram will be defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10-34 when expressed in the unit J s, which is equal to kg m2 s−1, where the metre and the second are defined in terms of c and ∆ν.
