Kilogram

For other uses of 'kg' see kg (disambiguation)

The kilogram or kilogramme, (symbol: kg) is the SI base unit of mass. It is defined as being equal to the mass of the international prototype of the kilogram.

It is the only SI base unit that employs a prefix http://www.bipm.org/en/si/history-si/name_kg.html, and the only SI unit that is still defined in relation to an artifact rather than to a fundamental physical property.

A kilogram is approximately equivalent to 2.205 avoirdupois pounds in the Imperial system and the customary system of weights and measures used in the United States.

History

The kilogram was originally defined as the mass of one litre of pure water at standard atmospheric pressure and at the temperature at which water has its maximum density (3.98 degrees Celsius). 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 dependency in the definition.

To avoid these problems, the kilogram was redefined as precisely the mass of a particular standard mass created to approximate the original definition. Since 1889, 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 is 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. It is accurate to state that any object in the universe (other than the reference metal in France) that had a mass of 1 kilogram 100 years ago, and has not changed since then, now has a mass of 1.000 000 050 kg. This perspective is counterintuitive and defeats the purpose of a standard unit of mass, since the standard should not change arbitrarily over time. The philosopher Saul Kripke elaborated on the philosophical implications of this kind of problem, referring, however, to the then-current definition of the metre in terms of an artifact, a choice which was later dropped.

The gram

The gram or gramme is the term to which SI prefixes are applied.

The reason the base unit of mass has a prefix is historic. Originally, the decimal system of units was commissioned by Louis XVI and in the original plans, the kilogram was supposed to be called the grave. A gramme was simply an alternative name for a thousandth of a grave, and a tonne for 1000 graves. However, the metric system didn't come in effect until after the French Revolution. At that time, the name "grave" had become politically incorrect, since it is an alternative word for the title "count" (cognate with the British "markgrave" and the German "Graf"), and nobility titles were not considered compatible with the notion of égalité.

The gram was also the base unit of the older CGS system of measurement, a system which is no longer widely used.

Proposed future definitions

There is an ongoing effort to introduce a new definition for the kilogram by way of fundamental or atomic constants. The proposals being worked on are:

Atom-counting approaches

The Avogadro approach attempts to define the kilogram as a fixed number of silicon atoms. As a practical realization, a sphere would be used and its size would be measured by interferometry.
The ion accumulation approach involves accumulation of gold atoms and measuring the electrical current required to neutralise them.

Fundamental-constant approaches

In a similar manner that the metre was redefined to fix the speed of light to an exact value of 299792458 m/s, there are proposals to redefine the kilogram in such a way to fix other physical constants of nature to exact values.

Electron mass: The kilogram is the base unit of mass, equal to 1 097 769 238 499 215 084 016 780 676 223 electron mass units.

This would have the effect of defining the electron mass to be m_e = 9.1093826 kg. This is consistent with the current 2002 CODATA value for the electron mass which is 9.1093826 ± 0.0000016 kg.

Planck's constant: The Watt balance uses the current balance that was formerly used to define the ampere to relate the kilogram to a value for Planck's constant, based on the definitions of the volt and the ohm. Using the Watt balance, a possible definition for the kilogram would be: The kilogram is the mass of a body at rest whose equivalent energy corresponds to a frequency of exactly (299792458)²/66260693 Hz.

This would have the effect of defining Planck's constant to be h = 6.6260693 J s. This is consistent with the current 2002 CODATA value for Planck's constant which is 6.6260693 ± 0.0000011 J s.

Elementary charge: Since the values of the Josephson constant (CIPM (1988) Recommendation 1, PV 56; 19) and von Klitzing constant (CIPM (1988), Recommendation 2, PV 56; 20) have proposed conventional values, it would be possible to combine these values (K_J-90 ≡ 4.835 979 Hz/V and R_K-90 ≡ 2.581 280 7 Ω) with the definition of the ampere to define the kilogram as follows: The kilogram is the mass which would be accelerated at precisely 2 m/s² if subjected to the per metre force between two straight parallel conductors of infinite length, of negligible circular cross section, placed 1 metre apart in vacuum, through which flow a constant current of exactly 6.241 509 629 152 65 elementary charges per second.

This would have the effect of defining the elementary charge to be e = 1.602176491612271 C. This is slightly different from but within the standard uncertainty of the current 2002 CODATA value for the elementary charge which is 1.60217653 ± 0.00000014 C. This is because the 1988 CIPM conventional values for the Josephson and von Klitzing constants are not identical to the 2002 CODATA values.

Link with weight

When the weight of an object is given in kilograms, the property intended is almost always mass. Occasionally the gravitational force on an object is given in "kilograms", but the unit used is not a true kilogram: it is the deprecated kilogram-force (kgf), also known as the kilopond (kp). An object of mass 1 kg at the surface of the Earth will be subjected to a gravitational force of approximately 9.80665 newtons (the SI unit of force). Note that the factor of 980.765 cm/s² (as the CGPM defined it, when cgs systems were the primary systems used) is only an agreed-upon conventional value (3rd CGPM (1901), CR 70) whose purpose is to define grams force. The local gravitational acceleration g varies with latitude and altitude and location on the Earth, so before this conventional value was agreed upon, the gram-force was only an ill-defined unit. (See also gee, a standard measure of gravitational acceleration.)

Examples

Attogram: a research team at Cornell University made a detector using NEMS cantilevers with sub-attogram sensitivity.
Yoctogram: can be used for masses of nucleons, atoms and molecules. It is a little large for light particles, but yocto- is the last official prefix in the sequence.
- The coefficient is close to the reciprocal of Avogadro's number: 1 unified atomic mass unit = 1.66054 yg
- Although the unified atomic mass unit is often convenient as a unit, one may sometimes want to use yoctograms to relate easily to other SI values.
- Mass of a free electron: 0.00091 yg
- Mass of a free proton : 1.6726 yg
- Mass of a free neutron: 1.6749 yg

SI multiples

Multiple	Name	Symbol	Multiple	Name	Symbol
10⁰	gram	g
10¹	decagram	dag	10^–1	decigram	dg
10²	hectogram	hg	10^–2	centigram	cg
10³	kilogram	kg	10^–3	milligram	mg
10⁶	megagram	Mg	10^–6	microgram	µg
10⁹	gigagram	Gg	10^–9	nanogram	ng
10¹²	teragram	Tg	10^–12	picogram	pg
10¹⁵	petagram	Pg	10^–15	femtogram	fg
10¹⁸	exagram	Eg	10^–18	attogram	ag
10²¹	zettagram	Zg	10^–21	zeptogram	zg
10²⁴	yottagram	Yg	10^–24	yoctogram	yg

When the Greek small letter mu (‘µ’) in the symbol of microgram is technically unavailable it should be replaced by Latin small letter ‘u’, but other informal abbreviations like ‘mcg’ (confusingly also used to designate the obsolete mongrel "millicentigram", equal to 10 µg) can also be encountered in practice. In the pharmaceutical industry, ‘mcg’ is used in the place of ‘µg’ to designate "microgram." The decagram is alternatively spelled ‘dekagram’.

The megagram is also more commonly known as the (metric) tonne (t), also spelled ton. This unit is accepted to be used with the SI and may take the same prefixes, see also metre-tonne-second system of units.

References

External links

SI base units | Units of mass

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