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Insights into Editorial: Full circle: on the change in kilogram’s definition

Insights into Editorial: Full circle: on the change in kilogram’s definition

Context: The definition of a kilogram has changed:

Scientists in 2018, have changed the way the kilogram is defined. The decision was made at the General Conference on Weights and Measures. The new definitions came into force on 20 May 2019.

The International Bureau of Weights and Measures (BIPM) had agreed to change the way we measured weight and as of the World Metrology Day (May20th), the kilogram has shed its 130-year-old definition to be more accurate and consistent.

General Conference on Weights and Measures (CGPM) is the highest international body of the world for accurate and precise measurements and comprises of 60 countries including India and 42 Associate Members.


Till now, the measurement of kilogram is:

Le Grand K has been at the forefront of the international system of measuring weights since 1889.

Several close replicas were made and distributed around the globe. But the master kilogram and its copies were seen to change – ever so slightly – as they deteriorated.

Currently, it is defined by the weight of a platinum-based ingot called “Le Grand K” which is locked away in a safe in Paris.

The wrong with Le Grand K is the fluctuation is about 50 parts in a billion, less than the weight of a single eyelash. But although it is tiny, the change can have important consequences.


What is the difference:

The problem is, the international prototype kilogram doesn’t always weigh the same. Even inside its three glass bell jars, it gets dusty and dirty, and is affected by the atmosphere. Sometimes, it really needs a wash.

So, when you just get it out of the vault, it’s slightly dirty. But the whole process of cleaning or handling or using the mass can change its mass. So, it’s not the best way, perhaps, of defining mass.

In the present system, you have to relate small masses to large masses by subdivision. That’s very difficult – and the uncertainties build up very, very quickly, said by a specialist in the engineering, materials and electrical science department at Britain’s National Physical Laboratory.


What’s needed is something more constant:

Just as the meter once the length of a bar of platinum-iridium, also kept in Paris is now defined by the constant speed of light in a vacuum, so a kilogram will be defined by a tiny but immutable fundamental value called the Planck constant.

The new definition involves an apparatus called the Kibble balance, which makes use of the constant to measure the mass of an object using a precisely measured electromagnetic force.

Electromagnets generate a force. Scrap-yards use them on cranes to lift and move large metal objects, such as old cars.

The pull of the electromagnet, the force it exerts, is directly related to the amount of electrical current going through its coils. There is, therefore, a direct relationship between electricity and weight.

One of the things this (new) technique allows us to do is to actually measure mass directly at whatever scale we like, and that’s a big step forward.

It had taken years of work to fine-tune the new definition to ensure the switchover will be smooth.

But while the extra accuracy will be a boon to scientists, for the average consumer buying flour or bananas, there will be absolutely no change whatsoever.


About Planck’s constant:

There is a quantity that relates weight to electrical current, called Planck’s constant – named after the German physicist Max Planck and denoted by the symbol h.

But “h” is an incredibly small number and to measure it, the research scientist Dr Bryan Kibble built a super-accurate set of scales.

The Kibble balance, as it has become known, has an electromagnet that pulls down on one side of the scales and a weight – say, a kilogram – on the other.

The electrical current going through the electromagnet is increased until the two sides are perfectly balanced.

By measuring the current running through the electromagnet to incredible precision, the researchers are able to calculate h to an accuracy of 0.000001%. This breakthrough has paved the way for Le Grand K to be deposed by “die kleine h”.

So, in principle, scientists can define a kilogram, or any other weight, in terms of the amount of electricity needed to counteract the weight (gravitational force acting on a mass).



The CSIR-NPL, which is India’s official reference keeper of units of measurements has released a set of recommendations requiring that school textbooks, engineering-education books, and course curriculum update the definition of the kilogram.

The institute is also in the process of making its own ‘Kibble Balance’, a device that was used to measure the Planck Constant and thereby reboot the kilogram.

In a world where accurate measurement is now critical in many areas, such as in drug development, nanotechnology and precision engineering, those responsible for maintaining the international system had no option but to move beyond Le Grand K to a more robust definition.