Dictionary Definition
User Contributed Dictionary
English
Alternative spellings
 (UK) decagramme
Etymology
Greek deka ten + gramTranslations
ten grams
 Greek: dekagramma
 Italian: decagrammo
Extensive Definition
The kilogram or kilogramme (symbol: kg) is the
base
unit of mass in the
International System of Units (known also by its
Frenchlanguage initials “SI”). The kilogram is defined as being
equal to the mass of the International Prototype Kilogram (IPK;
known also by its Frenchlanguage name Le Grand K), which
is almost exactly equal to the mass of one liter of water. It is the only SI
base unit with an SI prefix as
part of its name. It is also the only SI unit that is still defined
in relation to an artifact rather
than to a fundamental physical property that can be reproduced in
different laboratories. In everyday usage, the mass of an object in
kilograms is often referred to as its weight, although strictly
speaking the weight of an object is the gravitational force on it, measured in newtons (see also Kilogramforce).
Similarly, the avoirdupois pound, used
in both the Imperial
system and
U.S. customary units, is a unit of mass and its related unit of
force is the poundforce.
The avoirdupois pound is defined as exactly , making one kilogram
approximately equal to 2.205 avoirdupois pounds.
Many units in the SI system are defined relative
to the kilogram so its stability is important. After the
International Prototype Kilogram had been found to vary in mass
over time, the
International Committee for Weights and Measures (known also by
its Frenchlanguage initials CIPM) recommended in 2005 that the
kilogram be redefined in terms of fundamental constants of
nature.
The nature of mass
The kilogram is a unit of mass, the measurement of which corresponds to the general, everyday notion of how “heavy” something is. However, mass is actually an inertial property; that is, the tendency of an object to remain at constant velocity unless acted upon by an outside force. An object with a mass of one kilogram will accelerate at one meter per second squared (about onetenth the acceleration due to Earth’s gravity) when acted upon by a force of one newton (symbol: N).While the weight of matter is entirely
dependent upon the strength of local gravity, the mass of matter is
constant (assuming it is not traveling at a relativistic
speed with respect to an observer). Accordingly, for astronauts in
microgravity, no
effort is required to hold objects off the cabin floor; they are
“weightless.” However, since objects in microgravity still retain
their mass, an astronaut must exert ten times as much force to
accelerate a 10kilogram object at the same rate as a 1kilogram
object.
On 7 April 1795, the gram was decreed in France to be equal
to “the absolute weight of a volume of water equal to the cube of
the hundredth part of the meter, at the temperature of melting
ice.” Since trade and commerce typically involve items
significantly more massive than one gram, and since a mass standard
made of water would be inconvenient and unstable, the regulation of
commerce necessitated the manufacture of a practical
realization of the waterbased definition of
mass. Accordingly, a provisional mass standard was made as a
singlepiece, metallic artifact one
thousand times more massive than the gram—the kilogram.
At the same time, work was commissioned to
precisely determine the mass of a cubic decimeter (one liter) of water.
Although the decreed definition of the kilogram specified water at
0 °C—its highly stable temperature point—the scientists in
1799 chose to redefine the standard to water’s most stable density
point: the temperature at which water reaches maximum density,
which was measured at the time as 4 °C. They concluded
that one cubic decimeter of water at its maximum density was equal
to 99.92072% of the mass of the provisional kilogram made four
years earlier. That same year, 1799, an allplatinum kilogram
prototype
was fabricated with the objective that it would equal, as close as
was scientifically feasible for the day, the mass of a cubic
decimeter of water at 4 °C. The prototype was presented to
the Archives of the Republic in June and on 10 December 1799, the
prototype was formally ratified as the Kilogramme des Archive
(Kilogram of the Archives) and the kilogram was defined as being
equal to its mass. This standard stood for the next ninety
years.
International Prototype Kilogram
Since 1889, the SI system defines the
magnitude
of the kilogram to be equal to the mass of the International
Prototype Kilogram, often referred to in the professional metrology world as the “IPK”.
The IPK is made of an alloy of 90% platinum and 10% iridium (by weight) and is
machined into a rightcircular cylinder (height = diameter) of
39.17 mm to minimize
its surface area. The addition of 10% iridium improved upon the
allplatinum Kilogram of the Archives by greatly increasing
hardness while still
retaining platinum’s many virtues: extreme resistance to oxidation, extremely
high density,
satisfactory electrical
and thermal
conductivities, and low magnetic
susceptibility. The IPK and its six sister copies are
stored in an environmentally monitored safe in the lower vault
located in the basement of the
BIPM’s House of Breteuil in Sèvres on the
outskirts of Paris (see Links
to photographs, below for images). Three independently
controlled keys are required to open the vault. Official copies of
the IPK were made available to other nations to serve as their
national standards. These are compared to the IPK roughly every 50
years.
The IPK is one of three cylinders made in 1879.
In 1883, it was found to be indistinguishable from the mass of the
Kilogram of the Archives made eightyfour years prior, and was
formally ratified as the kilogram by the 1st
CGPM in 1889. This small, 25 ppm
difference, and the fact that the mass of the IPK was
indistinguishable from the mass of the Kilogram of the Archives,
speak volumes of the scientists’ skills over years ago when making
their measurements of water’s properties and in manufacturing the
Kilogram of the Archives.
Stability of the International Prototype Kilogram
The stability of the IPK is crucial because the kilogram underpins much of the SI system of measurement as it is currently defined and structured. For instance, the newton is defined as the force necessary to accelerate the kilogram by one meter per second squared. If the mass of the IPK were to change slightly, so too must the newton by a proportional degree. In turn, the pascal, the SI unit of pressure, is defined in terms of the newton. This chain of dependency follows to many other SI units of measure. For instance, the joule, the SI unit of energy, is defined as that expended when a force of one newton acts through one meter. Next to be affected is the SI unit of power, the watt, which is one joule per second. The ampere too is defined relative to the newton, and ultimately, the kilogram. With the magnitude of the primary units of electricity thus determined by the kilogram, so too follow many others; namely, the coulomb, volt, tesla, and weber. Even units used in the measure of light would be affected. The candela—following the change in the watt—would in turn affect the lumen and lux.Because the magnitude of many of the units
comprising the SI system of measurement is ultimately defined by
the mass of a yearold, golf ballsize
piece of metal, the quality of the IPK must be diligently protected
in order to preserve the integrity of the SI system. Yet, in spite
of the best stewardship, the IPK has likely already lost
µg relative to the average mass of the worldwide ensemble
of prototypes since the third periodic verification years
ago. Further, the world’s national metrology labs must wait for the
fourth periodic verification to confirm whether the historical
trends persisted.
Fortunately, definitions of
the SI units are quite different from their practical
realizations. For instance, the meter is defined as
the distance light travels in a vacuum during a time interval of of
a second. However, the meter’s practical realization typically
takes the form of a heliumneon laser, and the meter’s length is
delineated—not
defined—as wavelengths of light from this laser. Now suppose that
the official measurement of the second was found to have drifted by
a few parts per billion (it is actually exquisitely stable). There
would be no automatic effect on the meter because the second—and
thus the meter’s length—is abstracted via
the laser comprising the meter’s practical realization. Scientists
performing meter calibrations would simply continue to measure out
the same number of laser wavelengths until an agreement was reached
to do otherwise. The same is true with regard to the realworld
dependency on the kilogram: if the mass of the IPK was found to
have changed slightly, there would be no automatic effect upon the
other units of measure because their practical realizations provide
an insulating layer of abstraction. Any discrepancy would
eventually have to be reconciled though because the virtue of the
SI system is its precise mathematical and logical harmony amongst
its units. If the IPK’s value were definitively proven to have
changed, one solution would be to simply redefine the kilogram as
being equal to the mass of the IPK plus an offset value, similarly
to what is currently done with its replicas; e.g., “the kilogram is
equal to the mass of the
IPK + 42 ppb.”
The longterm solution to this problem, however,
is to liberate the SI system’s dependency on the IPK by developing
a practical realization of the kilogram that can be reproduced in
different laboratories by following a written specification. The
units of measure in such a practical realization would have their
magnitudes precisely defined and expressed in terms of fundamental
physical constants. While major portions of the SI system would
still be based on the kilogram, the kilogram would in turn be based
on invariant, universal constants of nature. While this is a
worthwhile objective and much work towards that end is ongoing, no
alternative has achieved the uncertainty of a couple parts in 108
(~20 µg) required to improve upon the IPK. However, as of
April 2007, the NIST’s implementation of the watt balance
was approaching this goal, with a demonstrated uncertainty of
36 µg. See Watt balance,
below.
Proposed future definitions
 In the following section, wherever numeric equalities are shown in ‘concise form’—such as —the two digits between the parentheses denotes the uncertainty at 1σ standard deviation (68% confidence level) in the two least significant digits of the significand.
The kilogram is the only SI unit that is still
defined in relation to an artifact. Note that the meter was also once
defined as an artifact (a single platinumiridium bar with two
marks on it). However, it was eventually redefined in terms of
invariant, fundamental constants of nature that are delineated via
practical realizations (apparatus) that can be reproduced in
different laboratories by following a written specification. Today,
physicists are investigating various approaches to do the same with
the kilogram. Some of the approaches are fundamentally very
different from each other. Some are based on equipment and
procedures that enable the reproducible production of new,
kilogrammass prototypes on demand (albeit with extraordinary
effort) using measurement techniques and material properties that
are ultimately based on, or traceable to, fundamental constants.
Others are devices that measure either the acceleration or weight
of handtuned, kilogram test masses and which express their
magnitudes
in electrical terms via special components that permit traceability
to fundamental constants. Measuring the weight of test masses
requires the precise measurement of the strength of gravity in
laboratories. All approaches would precisely fix one or more
constants of nature at a defined value. These different approaches
are as follows:
Atomcounting approaches
Carbon12
Though not offering a practical realization, this definition would precisely define the magnitude of the kilogram in terms of a certain number of carbon12 atoms. Carbon12 is an isotope of carbon. The mole is currently defined as “the quantity of entities (elementary particles like atoms or molecules) equal to the number of atoms in 12 grams of carbon12.” Thus, the current definition of the mole requires that (83⅓) moles of 12C has a mass of precisely one kilogram. The number of atoms in a mole, a quantity known as the Avogadro constant, is an experimentally determined value that is currently measured as being atoms (2006 CODATA value). This new definition of the kilogram proposes to fix the Avogadro constant at precisely and the kilogram would be defined as “the mass equal to that of 83⅓ · atoms of 12C.”Currently, the uncertainty in the Avogadro
constant is determined by the uncertainty in the measured mass of
12C atoms (a relative standard uncertainty of 50 parts per
billion at this time). By fixing the Avogadro constant, the
practical effect of this proposal would be that the precise
magnitude of the kilogram would be subject to future refinement as
improved measurements of the mass of 12C atoms become available;
electronic realizations of the kilogram would be recalibrated as
required. In an electronic definition of the kilogram, 83⅓ moles of
12C would—by definition—continue to have a mass of precisely one
kilogram and the Avogadro constant would continue to have
uncertainty in its precise value.
A variation on a 12Cbased definition proposes to
define the Avogadro constant as being precisely 84,446,8863 (≈)
atoms. An imaginary realization of a 12gram mass prototype would
be a cube of 12C atoms measuring precisely 84,446,886 atoms across
on a side. With this proposal, the kilogram would be defined as
“the mass equal to 84,446,8863 × 83⅓ atoms of 12C.” The
value 84,446,886 was chosen because it has a special property; its
cube (the proposed new value for the Avogadro constant) is evenly
divisible by twelve. Thus with this definition of the kilogram,
there would be an integer number of atoms in one gram of 12C:
50,184,508,190,229,061,679,538 atoms.
Avogadro project
This approach would define the kilogram as “the mass which would be accelerated at precisely when subjected to the permeter force between two straight parallel conductors of infinite length, of negligible circular cross section, placed 1 meter apart in vacuum, through which flow a constant current of (6,241,509,647,120,417,390) elementary charges per second”.Effectively, this would define the kilogram as a
derivative of the ampere,
rather than present relationship, which defines the ampere as a
derivative of the kilogram. This redefinition of the kilogram would
result from fixing the elementary
charge (e) to be precisely coulomb (from the current 2006
CODATA value of ), which effectively defines the coulomb as being
the sum of 6,241,509,647,120,417,390 elementary charges. It would
necessarily follow that the ampere then becomes an electrical
current of this same quantity of elementary charges per
second.
The virtue of a practical realization based upon
this definition is that unlike the watt balance and other
scalebased methods, all of which require the careful
characterization of gravity in the laboratory, this method
delineates the magnitude of the kilogram directly in the very terms
that define the nature of mass: acceleration due to an applied
force. Unfortunately, it is extremely difficult to develop a
practical realization based upon accelerating masses. Experiments
over a period of years in Japan with a superconducting,
30gram mass supported by diamagnetic
levitation never achieved an uncertainty better than ten parts per
million.
Magnetic hysteresis was one of the limiting issues. Other
groups are continuing this line of research using different
techniques to levitate the mass.
SI multiples
Because SI prefixes may
not be concatenated (serially linked) within the name or symbol for
a unit of measure, SI prefixes are used with the gram, not the kilogram, which
already has a prefix as part of its name. For instance,
onemillionth of a kilogram is 1 mg (one milligram), not
1 µkg (one microkilogram).
 When the Greek lowercase “µ” (mu) in the symbol of microgram is typographically unavailable, it is occasionally—although not properly—replaced by Latin lowercase “u”.
 The microgram is often abbreviated “mcg”, particularly in pharmaceutical and nutritional supplement labeling, to avoid confusion since the “µ” prefix is not well recognized outside of technical disciplines. Note however, that the abbreviation “mcg”, is also the symbol for an obsolete CGS unit of measure known as the “millicentigram”, which is equal to 10 µg.
 The unit name “megagram” is rarely used, and even then, typically only in technical fields in contexts where especially rigorous consistency with the units of measure is desired. For most purposes, the unit “tonne” is instead used. The tonne and its symbol, t, were adopted by the CIPM in 1879. It is a nonSI unit accepted by the BIPM for use with the SI. In English speaking countries it is usually called “metric ton”. Note also that the unit name “megatonne” or “megaton” (Mt) is often used in generalinterest literature on greenhouse gas emissions whereas the equivalent value in scientific papers on the subject is often the “teragram” (Tg).
Glossary
 Abstracted: Isolated and its effect changed in form, often simplified or made more accessible in the process.
 Artifact: A humanmade object used as a comparative standard in the measurement of a physical quantity.
 Check standard:

 A standard body’s backup replica of the International Prototype Kilogram (IPK).
 A secondary kilogram mass standard used as a standin for the primary standard during routine calibrations.
 Definition: A formal, specific, and exact specification.
 Delineation: The physical means used to mark a boundary or express the magnitude of an entity.
 Disseminate: To widely distribute the magnitude of a unit of measure, typically via replicas and transfer standards.
 IPK: Abbreviation of “International Prototype Kilogram” (CG image), the artifact which has a mass defined as precisely one kilogram.
 Magnitude: The extent or numeric value of a property
 National prototype: A replica of the IPK possessed by a nation.
 Practical realization: An artifact or readily reproducible apparatus to conveniently delineate the magnitude of a unit of measure.
 Primary national standard:

 A replica of the IPK possessed by a nation
 The least used replica of the IPK when a nation possesses more than one.
 Prototype:

 A humanmade object that serves as the defining comparative standard in the measurement of a physical quantity.
 A humanmade object that serves as the comparative standard in the measurement of a physical quantity.
 The IPK and any of its replicas
 Replica: An official copy of the IPK.
 Sister copy: One of six official copies of the IPK that are stored in the same safe as the IPK and are used as check standards by the BIPM.
 Transfer standard: An artifact or apparatus that reproduces the magnitude of a unit of measure in a different, usually more practical, form.
See also
 Inertia
 International System of Units (SI)
 International Bureau of Weights and Measures (BIPM)
 International Committee for Weights and Measures (CIPM)
 General Conference on Weights and Measures (CGPM)
 Gram
 Grave (orig. name of the kilogram, history of)
 Gravimetry
 Kilogramforce
 Metric system
 Mass
 Mass versus weight
 National Institute of Standards and Technology (NIST)
 Newton
 SI base units
 Standard gravity
 Tonne (metric ton)
 Watt balance
 Weight
Notes
External links
 National Institute of Standards and Technology (NIST): NIST Improves Accuracy of ‘Watt Balance’ Method for Defining the Kilogram
 The U.K.’s National Physical Laboratory (NPL): An overview of the problems with an artifactbased kilogram
 NPL: Avogadro Project
 NPL: NPL watt balance
 Metrology in France: Watt balance
 Australian National Measurement Institute: Redefining the kilogram through the Avogadro constant
 International Bureau of Weights and Measures (BIPM): Home page
 NZZ Folio: What a kilogram really weighs
 NPL: What are the differences between mass, weight, force and load?
 BBC: Getting the measure of a kilogram
decagram in Tosk Albanian: Kilogramm
decagram in Arabic: كيلوغرام
decagram in Aymara: Kilu
decagram in Bengali: কিলোগ্রাম
decagram in Min Nan: Kongkin
decagram in Belarusian: Кілаграм
decagram in Belarusian (Tarashkevitsa):
Кіляграм
decagram in Bosnian: Kilogram
decagram in Breton: Kilogram
decagram in Bulgarian: Килограм
decagram in Catalan: Quilogram
decagram in Czech: Kilogram
decagram in Danish: Kilogram
decagram in German: Kilogramm
decagram in Estonian: Kilogramm
decagram in Modern Greek (1453):
Χιλιόγραμμο
decagram in Spanish: Kilogramo
decagram in Esperanto: Kilogramo
decagram in Basque: Kilogramo
decagram in Persian: کیلوگرم
decagram in French: Kilogramme
decagram in Friulian: Chilogram
decagram in Irish: Cileagram
decagram in Galician: Quilogramo
decagram in Gan Chinese: 公斤
decagram in Korean: 킬로그램
decagram in Croatian: Kilogram
decagram in Indonesian: Kilogram
decagram in Interlingua (International Auxiliary
Language Association): Kilogramma
decagram in Icelandic: Kílógramm
decagram in Italian: Chilogrammo
decagram in Hebrew: קילוגרם
decagram in Javanese: Kilogram
decagram in Georgian: კილოგრამი
decagram in Swahili (macrolanguage):
Kilogramu
decagram in Latin: Chiliogramma
decagram in Latvian: Kilograms
decagram in Luxembourgish: Kilogramm
decagram in Lithuanian: Kilogramas
decagram in Lingala: Kilogálame
decagram in Hungarian: Kilogramm
decagram in Macedonian: Килограм
decagram in Marathi: किलोग्रॅम
decagram in Malay (macrolanguage):
Kilogram
decagram in Mongolian: Килограмм
decagram in Dutch: Kilogram
decagram in Japanese: キログラム
decagram in Norwegian: Kilogram
decagram in Norwegian Nynorsk: Kilogram
decagram in Uzbek: Kilogramm
decagram in Polish: Kilogram
decagram in Portuguese: Quilograma
decagram in Kölsch: Masse
decagram in Romanian: Kilogram
decagram in Russian: Килограмм
decagram in Scots: Kilogram
decagram in Albanian: Kilogrami
decagram in Sicilian: Chilu
decagram in Simple English: Kilogram
decagram in Slovak: Kilogram
decagram in Slovenian: Kilogram
decagram in Serbian: Килограм
decagram in SerboCroatian: Kilogram
decagram in Sundanese: Kilogram
decagram in Finnish: Kilogramma
decagram in Swedish: Kilogram
decagram in Tamil: கிலோகிராம்
decagram in Thai: กิโลกรัม
decagram in Vietnamese: Kilôgam
decagram in Ukrainian: Кілограм
decagram in Urdu: کلوگرام
decagram in Yiddish: קילאגראם
decagram in Contenese: 千克
decagram in Chinese: 千克