The Definition Of Kilogram Is Going To Change Forever

Finally, after hundred thirty years when it absolutely was established, the kilogram as we all know it’s on the brink of be retired. However it is not the end: on 20 may 2019, a replacement definition is going to be put in place – one that is much more correct than something we’ve had yet.

After the shift was unanimously voted in at the overall Conference on Weights and Measures in Versailles at the tip of last year, the modification is currently finally close to become official.

Most people do not think about science – the science of measurement – as we approach our day. However it’s immensely necessary. It isn’t simply the system by that we have a tendency to measure the world; it is also the system by that scientists conduct their observations. It must be precise, and it must be constant, ideally supported the laws of our Universe as we all know it.

But of the seven base units of the SI system of Units (SI), four don’t seem to be presently supported the constants of physics: the ampere (current), kelvin (temperature), mole (amount of substance) and kilogram (mass).


Therefore, Emeritus Director of the International Bureau of Weights and Measures (BIPM) Terry Quinn said that, “is that by having all the units based on the constants of physics, they are by definition stable and unaltering in the future, and universally accessible everywhere.

For example, a metre is decided by the gap light travels in an exceedingly vacuum in 1/299792458 of a second. A second is decided by the time it takes for a Cs atom to oscillate 9,192,631,770 times.

But, A kilo is outlined by… a kilo.

No, literally. it is a kilo weight referred to as the International prototype of the kilo (IPK), created in 1889 from 90 % platinum and 10 % iridium, and store in an exceedingly special vault within the BIPM headquarters.

In fact, the kilo is that the solely base unit within the SI still outlined by a object.

There are copies of the IPK in numerous locations round the world, that are used as national standards and sometimes sent back to France to be compared against the prototype.

And that’s wherever things get fascinating – the mass of those copies has been discovered to be drifting far from that of the IPK bolted away within the vault. It’s unclear whether or not the copies were losing mass or the IPK was gaining mass, however neither situation is good for scientific exactitude, although we’re addressing mere micrograms.


For the previous few years, metrologists are talking regarding the necessity for a brand new standard. Now, they are finally able to redefine the kilogram supported the physicist constant, the ratio of energy to frequency of a photon, measured to its most precise value.

Now it’s possible to define the kilogram in terms of a constant of physics – the Planck constant, the speed of light and the resonant frequency of the caesium atom.

Why all three?

This is because the units of the Planck constant are kgm2s-1, so we need first to have defined the meter (in terms of the speed of light) and the second (in terms of the caesium atom in the atomic clock).

So below the new definition, the magnitude of a kilogram would be,”set by fixing the numerical value of the Planck constant to be equal to exactly 6.626 069… × 10–34 when it is expressed in the SI unit kgm2s-1, which is equal to J s“.


While it won’t make a difference  to most people’s lives at all – a kilogram of apples before the change is still going to be a kilogram of apples after the change – but it will make a difference to metrologists in particular, and scientists in general.

It will be the end of an era, really – and also the start of a new one.

As for the IPK itself, the tiny piece of metal that has been so necessary for so many years can still be kept within the same conditions it continuously has, beneath two bell jars in an exceedingly climate-controlled vault.

That’s part to honour its legacy; however scientists will forever be scientists. It’ll also be studied in future years and decades to observe what proportion its mass changes. Now against the new, changeless definition of the kilogram. Finally we’ll be able to tell obviously if it’s truly been losing mass all now.

Whereas it should look complicated, the new system will truly be simply understood by anyone.

The new kilogram definition will come into effect on World Metrology Day: 20 May 2019.