For centuries, scientists have struggled with how to define the kilogram. The reason behind this quest for a definition is, unlike other measurements, the kilogram is still defined by a physical artifact, “the international prototype kilogram or IPK.” The kilogram was originally determined in layman’s terms as a cubic liter of ice water, and this was named grave, which derives from the Latin word "gravitas," meaning weight. This measurement was thought to be too big so they made it smaller by introducing the gram which is one thousandth of the original cubic liter. They soon discovered this measurement was too small for practical everyday use so they renamed the grave to kilogram or 1000 grams. However, water does fluctuate in temperature and density very rapidly so it was not the best basis of which to name a unit of measurement. A pure platinum cylinder was soon created and had the same mass as the water from the original definition. This was dubbed “kilogram of the archives." Thus, the kilogram was no longer defined as the mass of a volume of water, the “kilogram of the archives” is by definition “the kilogram.” Later this “kilogram of the archives” was upgraded in substance to a platinum iridium alloy which, is much harder than the original but still relatively identical. As of today this is the definition of the kilogram. This physical object was then replicated and sent out all over the globe so various nations would all have the definition of the kilogram for reference. They are all stored in similar environments and kept locked away so they are not altered by humans or the atmosphere. However, at periodic weigh-ins, scientists discovered the masses of all of the various replicas had altered. Even though all of the kilograms were made of the same alloy and stored under the almost identical conditions the masses varied ever so slightly to the original. This meant some physical change over time had to of taken place. Though this is still in debate one thing that can be certain is that platinum iridium is not stable over time and subject to change.
This is where the world’s roundest object comes into play. The project is named The Avogadro Project and the goal is to redefine Avogadro’s constant. Avogadro’s Constant is currently the number of atoms in 12g of Carbon-12. "[The aim is to] reverse the relationship so the kilogram is precisely specified by Avogadro's constant. By creating an almost perfect sphere that is composed of a single crystal of silicon 28 atoms. By carefully measuring the diameter, the volume can be precisely specified. Since the atom spacing of silicon is well known, the number of atoms in a sphere can be accurately calculated" (Veritasium). This then allows one to redefine Avogadro’s constant. Avogadro’s constant is equal to the number of atoms of 12 grams of C-12. If we were to use the number of atoms calculated in the silicon 28 sphere it would then be used to fix Avogadro’s constant which would then in turn define the kilogram. In conclusion, even if the spheres were lost or damaged, "it would have no effect on the definition of the kilogram because it would now be defined not by an object but by a concept" (Veritasium). So if this were widely accepted as the new definition of a kilogram would be the mass of 2.5 x 1025 silicon-28 atoms. Thus, the kilogram will become an unchanging unit and a solid concept of a unit.
Submitted by Sam Krause
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