No One Knows How Much a Kilogram Actually Weighs

June 29, 2016

Something you may consider consistent in day-to-day life is actually based on a prototype that is inevitably changing: that something being the official kilogram.

All units of measurement are based on a concept, a Mach is the speed of sound, a degree is based on the properties of water, and a kilogram which is based off a single prototype kept secure in Paris, and it is constantly changing.

The kilogram’s long history started about the end of the 1700’s when King Louis XVI of France ordered a new unit of measurement in order to take action against cheaters and swindlers alike. It was decided by the King’s Commission that the new unit of measurement would become the decimal metric system. The new measurement of mass was proposed to be called a grave, defined as the mass of one liter of water at its freezing point.

Following the French revolution, the New Republic decided to adopt the new metric system. The Republic decided to make a few small amendments to the system defining the new standard of mass as the gram- the absolute mass of 1 cm³ of water at 4 °C. However, the system became extremely impractical for commercial use since the measurement was many times smaller than most products- about the size of a pea. They then decided on something more practical and to the liking closer to a grave. It was decided the new unit was to be one thousand times bigger than a gram, becoming the kilogram.

It was on June 22, 1799 where the Republic constructed the first official mass artifact based on their system of measurement. The mass was forged out of platinum and was dubbed the “kilogram of the archives”- the one and only true mass of exactly one kilogram. However with global economies expanding through the 1800’s, the inconsistent units of measurements became a persistent problem.

Each country possessed their own definition of a single unit of measurement, causing much controversy and difficulties in conversions. In 1875, 17 countries set to resolve the issue by signing a pact, a single system for units of measurement. It was decided that the Metric system was to be adopted. The official unit of a kilogram was remade as a new standard based off of the old, pure platinum prototype. The new prototype was made from alloy of platinum (90%) and iridium (10%), and dubbed the International Prototype Kilogram (IPK).

kilogramThe Official International Prototype Kilogram (IPK) [Image Source: BIPM]

A few countries received copies of the IPK, however, by definition the official measurement remains with the original copy. No matter what happens to the IPK, it remains exactly one kilogram. The IPK varies every year by a few micrograms as dust and radiation slightly alter its composition, thereby altering its mass. As a result, the kilogram is constantly changing. Every scale and measuring device is measured against a copy of a copy of a copy that eventually results in a comparison against the original kilogram. But since the kilogram is always changing, every device is slightly miscalibrated.

However, International Committee on Weights and Measures is in current discussions of formalizing the measurement to give an exact, indisputable measurement of exactly one kilogram. The solution is likely to be proposed off of a seemingly unlikely solution. Speakers, set at different tones varied by voltage can actually be used to determine mass. As a current passes through the coils  in a speaker, the cone undergoes displacement. A mass can be placed on a speaker which slightly compresses the cone.  Through a series of deductions, it can be concluded exactly how much the mass weighs. It is likely in the near future the kilogram may change again and be based off of a more scientific, and sound unit.

The current “SI units”, including kilogram, the ampere, the kelvin and the mole will undergo changes and will fall under a new definition to standardize all units of measurement as constants. Whether they are based off of standardized experiments or defined in terms of constants, the units of measurements will once again change.

However, as of now, the kilogram is forever changing, making every mass around the world, according to our definition, change as well.

SEE ALSO: Why the Imperial System of Measurement is the Worst