‘One Kilogram’ Is Getting A Whole New Definition From Today
Scientists on Friday unanimously voted to redefine kilogram, retiring the platinum alloy cylinder conceived in Paris in 1889. Redefined by the Planck constant, the new system determines the unit of mass through the electrical force needed to counteract the weight of a kilogram on a Kibble balance. The changes will come into effect from May 20, 2019, World Metrology Day.
You see, back in 1889, the kilogram was standardised so that there would be no confusion across the world. This standard was set up by having a block of platinum-iridium machined into the shape of a cylinder. This gold standard was called the “international prototype kilogram”, or “Le Grande K”. There have since been exact replicas of it distributed to other countries, so they can standardise their own weights as well.
However, even kept behind three glass bell jars in a vault in Paris, Le Grande K can get dusty, meaning it’s fractional weight changes. And there’s only so many times you can take it out and wipe it down before you have to find a better solution
That’s what’s happening today, with scientists and measurement experts meeting at the ‘International Bureau of Weights and Measures to implement their solution. They’re creating a new “electronic kilogram”, one that can’t be contaminated by dust or deterioration.
How it’ll happen
Electromagnets generate an attractive force, which is directly correlational to the amount of electrical current going through its coils. So technically, you can define a kilogram by how much electrical current is needed for an electromagnet to exactly counteract the weight.
However, the weight to electrical current measure is something called Planck’s constant (denoted as h), which is an incredibly small number. In order to measure it exactly, these scientists have to use an apparatus called a Kibble balance. One side of the balance scales has an electromagnet pulling down, while a weight is put in the other. You get an exact measurement when the scales are perfectly balanced.
With this method, scientists can calculate Planck’s constant (and therefore weight) to an accuracy 0.000001 percent. Which is why they’re now satisfied it can be used to replace Le Grande K.
Until now, replica kilograms across the world have been checked against Le Grande K every few decades, and been found to be off by about 50 parts to a billion. With the electronic kilogram though, there will be no “master copy” to check against, just a number. That way anyone, anywhere in the world with a Kibble balance can check their weights.
That’s huge for scientists, when accurate measurements are crucial to their studies. Regular people like you and me though? We’re not going to notice a thing.