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The kilogram (symbol: kg) is the SI base unit of mass. A gram is defined as one thousandth of a kilogram. Conversion of units describes equivalent units of mass in other systems.1 Multiples
SI prefixes are used to name multiples and subdivisions of the kilogram. The most commonly used ones are:
- tonne = 1 000 kilograms (strictly speaking, this should be named megagram, but the name is rarely used) (not to be confused with the US short ton, 2000 pounds or about 907 kilograms)
- gram = 1/1 000 kilogram
- milligram = 1 thousandth of a gram = 1 millionth of a kilogram
- microgram = 1 millionth of a gram = 1 billionth of a kilogram
2 Definition
The kilogram is the only one of the SI units which is still defined in relation to an artifact rather than to fundamental physical properties.
The kilogram was originally defined as the mass of one litre of pure water at a temperatureTemperature is the physical property of a system which underlies the common notions of "hot" and "cold"; the material with the higher temperature is said to be hotter. General description The formal properties of temperature are studied in thermodynamics. of 4 degrees CelsiusThe degree Celsius (°C) is a unit of temperature named after the Swedish astronomer Anders Celsius ( 1701 1744), who first proposed it in 1742. The Celsius temperature scale was designed so that the freezing point of water is 0 degrees, and the boiling po and standard atmospheric pressure.
This definition was hard to realize accurately, partially because the density of water depends ever-so-slightly on the pressure, and pressure units include mass as a factor, introducing a circular dependencyA circular definition is one that assumes a prior understanding of the term being defined. For instance, we can define " oak" as a tree which has catkins and grows from an acorn, and then define "acorn" as the nut produced by an oak tree. To someone not k in the definition of the kilogram.
To avoid these problems, the kilogram was redefined as precisely the mass of a particular standardThe word standard has several meanings: Classically, standard referred to a flag or banner; especially, a national or other ensign carried into battle; thus "standard bearer" indicates the one who bears, or carries, the standard. The modern primary meanin mass created to approximate the original definition. Since 1889Events January-April January 8 Herman Hollerith receives a patent for his electric tabulating machine January 22 Columbia Phonograph is formed in Washington, DC. February 11 Meiji Constitution of Japan adopted; 1st Diet convenes in 1890 January 30 ? Crown, the SI system defines the unit to be equal to the mass of the international prototype of the kilogram, which is made from an alloy of platinum and iridium of 39 mm height and diameter, and kept at the Bureau International des Poids et Mesures (International Bureau of Weights and Measures). Official copies of the prototype kilogram are made available as national prototypes, which are compared to the Paris prototype ("Le Grand Kilo") roughly every 10 years. The international prototype kilogram was made in the 1880s.
By definition, the error in the repeatability of the current definition is exactly zero; however, in the usual sense of the word, it can be regarded as of the order of 2 micrograms. This is found by comparing the official standard with its official copies, which are made of roughly the same materials and kept under the same conditions. There is no reason to believe that the official standard is any more or less stable than its official copies, thus giving a way to estimate its stability. This procedure is performed roughly once every forty years.
The international prototype of the kilogram seems to have lost about 50 micrograms in the last 100 years, and the reason for the loss is still unknown (reported in Der Spiegel, 2003 #26). The observed variation in the prototype has intensified the search for a new definition of the kilogram. Although it is accurate to state that all other objects in the universe have gained 50 micrograms per kilogram, this perspective is counterintuitive and defeats the purpose of a standard unit of mass.
