Quest to find the Perfect Kilogram! (Le Grand K)

The kilogram is stored under three bell jars, next to six sister kilograms, in a climate-controlled vault locked by three independently controlled keys, in the basement of the International Bureau of Weights and Measures on the outskirts of Paris.

That’s right fellas, the SI unit of mass which we all have been using since, like ever, is a thing, an object. In fact it is the only unit which is defined with an object as it’s basis. This means if you were to break into the vault and tamper with the kilogram or ‘#The_Big_K’, you’ll be changing the definition of kilogram!


Grave, I don’t know how, but those people knew a lot about puns. Grave was the name of the base unit of mass in the metric system, which became Systeme International d’unites or the SI units.

In 1793, a commission which included notable scientist and aristocrat Antoine Lavoisier, defined the base unit of mass as the weight of a cubic decimetre of water at the melting temperature of ice.

Antoine Lavoisier lost his head later, not because of this though, he was convicted of collecting taxes.

This measure was too big, according to the republican government, so they settled for gramme, which soon they realised was too small, so they returned to the grave,but now called it as kilogram. And that is why out of 7 base SI units, only kilogram has a prefix.

In 1799, the kilogram definition was redefined to be the mass of 1 litre of water at 4 degrees Celcius.

Water wasn’t a reliable thing as it’s density varies, so a platinum cylinder was designed to have the same mass as the water at those conditions, and was named as the ‘kilogram of the archives’.

Hence the kilogram of the archives was the kilogram itself.

Later it was upgraded to platinum-iridium alloy cylinder, and to this day, it remains the official definition of the kilogram, or otherwise known as ‘International Prototype Le Grand K’

And it’s the only thing in the universe with mass of exactly one kilogram.

How do we know it weighs exactly one kilogram

When the Big K was created, other 40 pieces identical to it were created too. These were sent to other countries to serve the purpose. But when these were united back, their were deviations from the original weight these had, even the Big K didn’t have the same mass as other 6 sisters it was stored with. So, we cannot, because it is changing. But the kilogram is a major SI unit, as 4 out of 7 SI unit depends on it, and other derived units too.


There are two solutions, which are being discussed.

1. Fixing Avogadro’s number by using The perfect silicon-28 crystal

2. Fixing Plank’s constant by using Watt Balance

These two will adjust the concept of kilogram and it’ll not be defined by an object.

For example, right now Avogadro’s constant is defined on the basis of kilogram. It is equal to the number of atoms in 12 grams of carbon-12. But using the silicon crystal, knowing the exact amount of atoms in it and also it’s diameter to an unmatched precision, kilogram can be redefined as ‘It is the mass of 2.15×10^25 silicon-28 atoms’.

It is highly likely that these new definitions might be accepted. But till then we will be using the “Le Grand K” as the Kilogram!


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s