Published: Sun, November 18, 2018
Worldwide | By Isabel Fisher

Kilogram, Amp and Kelvin Redefined in Largest Units Overhaul Since 1875

Kilogram, Amp and Kelvin Redefined in Largest Units Overhaul Since 1875

To make a milestone in the science of measurement, representatives from 60 nations voted to redefine the International System of Units (SI), changing the word's meaning of the kilogram, the ampere, the Kelvin and the mole, until the end of time.

Pharmaceutical companies are now measuring mass at microgram levels, which is only a billionth of a kilogram. Gathering this week in Versailles, west of Paris, governments on Friday November 16, 2018, are expected to approve a plan to instead use a scientific formula to define the exact weight of a kilo. Many decades ago, scientists began to demand a more precise, more stable measurement system.

So the plan is to redefine the kilogram by tying it to a fundamental feature of the universe called Planck's constant.

Dr Theodoor Janssen, director of research at the National Physical Laboratory in Teddington, south-west London, which is responsible for the UK's measurement standards, said: "The SI redefinition is a landmark moment in scientific measurement".

In May 2019 when the revised definition of the kilogram is implemented, it will be based on three fundamental constants: the Planck constant, the speed of light and the cesium atom's natural microwave radiation.

Currently, there is only one true kilogram, known as Le Grand K, which is secured in a vault outside Paris and used to calibrate all measures of mass throughout the world.


The definition of the kilogram for more than 130 years, the International Prototype of the Kilogram (IPK), a cylinder of a platinum alloy stored at the BIPM in France, will now be retired.

"We know from comparing the kilogram in Paris with all the copies of the kilogram that are all around the world that there are discrepancies between them and Le Grand K itself", Dr Stuart Davidson, head of mass spectrometry at the National Physical Laboratory in the United Kingdom told the BBC.

The new system will allow us to determine the pounds of anyone who has a device called the balance of the Kibble.

Kibble balance is an electromechanical weighing machine (see photo on top) that gives accurate results.

After a week-long meeting at the Palace of Versailles, outside Paris, BIPM members made a decision to "bring an end to the use of physical objects to define measurement units", and update definition for the kilogram in terms of constants that "will open the opportunity for the use of new technologies, including quantum (ones)".

"The current kilogram mass exerts a specific amount of force in Earth's gravity", NIST said. This was because the machine measures both current and voltage, which generates power that is denoted in watts. The mass of an object is found from the equality of electric and mechanical forces. "It's about as excited as you're going to see metrologists get", David Newell a researcher at the National Institute of Standards and Technology, told Science News. This mass was then represented by a cylinder - the prototype that was adopted as a unit of weight by other countries.

Like this: