How is accuracy defined?

Le Grand K
Back in the day, people in different countries used various different measurement units in daily life. This became a problem as trading grew more and more common across borders.
Reference units were created and defined, like a platinum-based ingot known as "Le Grand K" that would be the reference for the kilogram. A special laboratory (Bureau International des Poids et Mesures) was founded in France, to take care of various measurement references. They manufactured national-level reference pieces, which were copies of the original international pieces. These national-level pieces would then be copied again for those countries to use.
There were a few problems with this system - first, the national-level reference pieces needed to be physically brought all the way to Paris for regular recalibration.
But more importantly, because the kg was based on a physical object, its survival wasn't guaranteed. It had been made in 1889, and the master kilogram and its copies were observed to change - nearly imperceptibly , deteriorating with time. Thus while it was still suitable at the time, it wouldn't be in the future. Alternatively, they could possibly be destroyed in war, or an accident!
The official definition reads thus: "The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram."
So to avoid relying upon the stability of a single man-made object to define key measurements, the idea was to find something invariant in nature that would never change. It would replaced using an electrical measurement, that has been determined to be more stable, accurate, and egalitarian.
What is the new method?
Electromagnets generate force, the strength of which his directly related to the amount of current going through its coals. Therefore, a direct relationship between electricity and weight could be established! In principle, what scientists did was define the kilogram (or other weight) in terms of the amount of electricity needed to counteract the weight. The new system, once fully adopted worldwide, allows anyone with a "Kibble Balance" (which is essentially an electromagnet that pulls down on one side of the scale, and a weight on the other) to check their weights much more easily compared to before.

Kibble Balance

There's certainly more to accurate weighing than may meet the eye! When you use a certified scale, you know that the results have been proven and confirmed. In health care or other fields of application where body measurement is used for important functions such as medication dosage, accuracy is of particular importance!

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