We think we know what a kilogram is, but do we really? While it seems to be a simple question, the definition of a kilogram has changed multiple times since its invention in the late 1700s. The kilogram is a base unit of mass in the metric system. Therefore, there must be no confusion as to what a kilogram is. Everyone must be able to agree on its precise mass, especially scientists and engineers when making important calculations. In the late 1700s, one kilogram was defined by the mass of one litre of water, but this approach had a practical problem. [ihc-hide-content ihc_mb_type=”show” ihc_mb_who=”2,3″ ihc_mb_template=”1″ ]
For accuracy, it needed to be weighed in a vacuum to remove any influence of air pressure on mass, which was very difficult to achieve. The kilogram was different every time the water was re-weighed. In 1889, a new standard for the kilogram was formally ratified. This time, it was in the form of a platinum-iridium cylinder named the International Prototype of the Kilogram, or the IPK.
The IPK is fiercely protected under a series of glass jars and in a secure vault in the outskirts of Paris. Unfortunately, the IPK’s mass seemed to be changing. In one theory, the IPK’s routine cleaning was responsible for a loss in mass over time. Since May 2019, we officially have a newer and better kilogram in town, which is no longer defined using a physical object. Rather, the present kilogram is more accurate because it is based on fixed constants in nature, including the Planck constant, the speed of light, and properties of the Caesium-133 atom. With the proper equipment, anyone would be able to calculate what a kilogram is, and the result would always be exactly the same.[/ihc-hide-content]
