A KILO IS A KILO IS A KILO, RIGHT? WRONG!
Monday marks World Metrology Day, and this year's edition sees a big change in the way the kilogram unit is defined.
In November last year, scientists and policy makers from around 60 nations voted unanimously to redefine the kilogram, and Monday is the day their decision takes effect.
The new definition is based on the Planck constant -- a physical constant observed in the natural world -- rather than the precise weight of a piece of metal kept under lock and key.
For more than 100 years Paris has been home to Le Grand K -- or the International Prototype Kilogram, as it is officially known -- a block of metal that previously defined the weight of a kilogram.
Until now, everything from kitchen scales to gym weights around the world was manufactured to the standard set by the cylinder of platinum iridium, which has been kept in a high-security vault in the French capital since 1889.
The new definition of the kilogram will be based on the Planck constant.
Different countries have their own "prototype kilograms" that serve as national standards, which were calibrated to the Paris artifact.
The reason for the change to the International System of Measurement (SI) units was that over time the prototype lost atoms and therefore mass because it is "susceptible to damage and environmental factors," according to the National Physical Laboratory (NPL), which houses Kilo 18, Britain's copy of Le Grand K.
Le Grand K was compared with the various copies only once every 40 years, which made the calibration potentially inaccurate. Though the change in mass is roughly equivalent to the weight of an eyelash, the repercussions could be severe.
"This is fine when it comes to measuring a bag of sugar, but is becoming unacceptable for more sophisticated science, such as when measuring doses in pharmaceuticals," a statement from the NPL said, and Le Grand K will now enter retirement.
The new definition is based on the Planck constant, which is inherently stable, according to the NPL.