Definition of kilogram redefined: How will it affect the consumers?

On November 16, 2018, at the General Conference on Weights and Measures, in Versailles, Paris, the definition of kilogram was changed forever.
On November 16, 2018, at the General Conference on Weights and Measures, in Versailles, Paris, the definition of kilogram was changed forever.

t’s official. The wait is finally over!

In a historic judgment, more than 60 nations came together to vote unanimously for a new system that redefines the kilogram along with three other units, the ampere (electrical current), the kelvin (thermodynamic temperature) and the mole (amount of a substance).

On November 16, 2018, at the General Conference on Weights and Measures, in Versailles, Paris, the definition of kilogram was changed forever.

How was it defined before?

The kilogram has been defined since 1889 by a shiny piece of platinum-iridium kept in a special glass case, the International Prototype of the Kilogram (IPK), also known as Le Grand K.

It is housed at the headquarters of the International Bureau of Weights and Measures (BIPM), just outside Paris.

All modern mass measurements are traceable back to it – from micrograms of pharmaceutical medicines to kilos of apples and pears and tonnes of steel or cement.

Why is the kilogram redefined?

The problem is, the IPK doesn’t always weigh the same. Even inside its three glass bell jars, it picks up microparticles of dirt and is affected by the atmosphere. Sometimes it needs cleaning, which can affect its mass.

That can have profound implications. If the prototype were to lose mass, atoms would, in theory, weigh more since the base kilogram must by definition always weigh a kilogram.

“We live in a modern world. There are pollutants in the atmosphere that can stick to the mass,” said Ian Robinson, a specialist in the engineering, materials and electrical science department at Britain’s National Physical Laboratory.

“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.”

What’s needed is something more constant.


How will it be defined now?

A kilogram will be defined by a tiny but immutable fundamental value called the ‘Planck constant.’

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

“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,” said Robinson.

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.

It is arguably the most significant redefinition of an SI unit since the ‘second’ was recalculated in 1967, a decision that helped ease communication across the world via technologies like GPS and the internet.

The history 


The reason why “kilogram” is the name of a base unit of the SI is an artefact of history.

Louis XVI charged a group of savants to develop a new system of measurement. Their work laid the foundation for the “decimal metric system”, which has evolved into the modern SI. The original idea of the king’s commission (which included such notables as Lavoisier) was to create a unit of mass that would be known as the “grave”. By definition it would be the mass of a litre of water at the ice point (i.e. essentially 1 kg). The definition was to be embodied in an artefact mass standard.

After the Revolution, the new Republican government took over the idea of the metric system but made some significant changes. For example, since many mass measurements of the time concerned masses much smaller than the kilogram, they decided that the unit of mass should be the “gramme”. However, since a one-gramme standard would have been difficult to use as well as to establish, they also decided that the new definition should be embodied in a one-kilogramme artefact. This artefact became known as the “kilogram of the archives”. By 1875 the unit of mass had been redefined as the “kilogram”, embodied by a new artefact whose mass was essentially the same as the kilogram of the archives.

The decision of the Republican government may have been politically motivated; after all, these were the same people who condemned Lavoisier to the guillotine. In any case, we are now stuck with the infelicity of a base unit whose name has a “prefix“.


Please enter your comment!
Please enter your name here