A rendering of the platinum-iridium cylinder, that currently defines the weight of a kilogram (Image: Greg L)
It’s one of those things where if you think about it too much, your head might explode... we know there are 1,000 grams in a kilogram, and 1,000 kilograms in a metric ton, but how was it ever decided what any of these units actually physically weighed? Well... the modern metric system is part of the Système International d'Unités (International System of Units) or SI. It states that a kilogram is the weight of one specific 130 year-old platinum-iridium cylinder, which is kept in a vault at the International Bureau of Weights and Measures in France... and no, don’t ask how they knew if they’d got its weight right, when they were making it. The problem is, that cylinder’s mass changes slightly over time. Now, a worldwide effort is under way to change the definitive weight of a kilogram to something more permanent.
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