The kilogram was first defined by the mass of one litre of water in the year 1795 during the French Revolution. This definition specified the mass of one litre of water at the temperature of its maximum density, which is approximately 4 degrees Celsius. The French National Convention decreed that the gram would be the mass of one cubic centimetre of water at the melting point of ice.
The International Prototype of the Kilogram diverged in mass from its official copies over time, losing approximately 50 micrograms since their manufacture in the late 19th century. This drift occurred because the platinum and iridium cylinder was not immune to change, creating a crisis in metrology. Scientists suspected surface contamination or cleaning methods were the culprit, but the mystery remained unsolved for over a century.
The redefinition of the kilogram based on the Planck constant took effect on the 20th of May 2019. The International Committee for Weights and Measures approved the resolution in 2018 to fix the value of the Planck constant to exactly 6.62607015 times 10 to the minus 34 joule-seconds. This decision removed the need for a physical object to define mass and replaced the metal cylinder with a mathematical constant.
The Kibble balance realizes the new definition of the kilogram by using the principles of electromagnetism to balance the weight of a mass against the force generated by an electric current. This machine converts a weight measurement into a mass by expressing the magnitude in electrical terms via special components that permit traceability to physical constants. The process requires precise measurement of the strength of gravity in laboratories to ensure that the weight of the test mass is accurately determined.
The kilogram is the only base unit that includes an SI prefix as part of its name because the word is derived from the French kilogramme, which prefixes the Greek stem of kilo to gram. The term gram comes from the Greek word gramma, which originally meant something written or a letter, but came to be used as a unit of weight. The French spelling was adopted in Great Britain when the word was used for the first time in English in 1795.
The abbreviation mcg rather than the SI symbol μg is formally mandated for medical practitioners in the United States by the Joint Commission on Accreditation of Healthcare Organizations to prevent errors. In the United Kingdom, the National Institute for Health and Care Excellence and Scottish Palliative Care Guidelines state that micrograms and nanograms must both be written in full, and never abbreviated as mcg, μg or ng respectively. These guidelines reflect the critical importance of clarity in medical dosing where a single misplaced decimal point or misinterpreted abbreviation can lead to fatal consequences.