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Encyclopedia > Kilogram
Shown above is a computer-generated image of the International Prototype Kilogram (“IPK”). The IPK is the kilogram. It sits next to an inch-based ruler for scale. The IPK is made of a platinum-iridium alloy and is stored in a vault at the BIPM in Sèvres, France. For other kilogram-related images, see Links to photographs, below.
Shown above is a computer-generated image of the International Prototype Kilogram (“IPK”). The IPK is the kilogram. It sits next to an inch-based ruler for scale. The IPK is made of a platinum-iridium alloy and is stored in a vault at the BIPM in Sèvres, France. For other kilogram-related images, see Links to photographs, below.

The kilogram or kilogramme (symbol: kg) is the SI base unit of mass. The kilogram is defined as being equal to the mass of the International Prototype Kilogram (IPK), 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. KG, Kg or kg may indicate: A symbol for kilogram (always kg) A symbol for kilogauss (always kG) A Kampfgeschwader, a bomber squadron of the former German Luftwaffe Volkswagen Abbreviation in the VW community for Karmann Ghia King George County, Virginia The Klein-Gordon equation of particles in quantum mechanics... Image File history File links Metadata Size of this preview: 800 × 585 pixelsFull resolution (1400 × 1024 pixel, file size: 523 KB, MIME type: image/jpeg) 19:00, August 10, 2007 . ... Image File history File links Metadata Size of this preview: 800 × 585 pixelsFull resolution (1400 × 1024 pixel, file size: 523 KB, MIME type: image/jpeg) 19:00, August 10, 2007 . ... General Name, Symbol, Number platinum, Pt, 78 Chemical series transition metals Group, Period, Block 10, 6, d Appearance grayish white Standard atomic weight 195. ... This article is about the chemical element. ... The Bureau International des Poids et Mesures (International Bureau of Weights and Measures, or BIPM) is a standards organization, one of the three organizations established to maintain the SI system under the terms of the Metre Convention. ... Road to Sèvres, Jean-Baptiste Camille Corot, 1855-1865. ... “SI” redirects here. ... The SI system of units defines seven SI base units: physical units defined by an operational definition. ... For other uses, see Mass (disambiguation). ... The litre or liter (see spelling differences) is a unit of volume. ... An SI prefix (also known as a metric prefix) is a name or associated symbol that precedes a unit of measure (or its symbol) to form a decimal multiple or submultiple. ...


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 Kilogram-force). 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 pound-force. The avoirdupois pound is defined as exactly 0.453 592 37 kg, making one kilogram approximately equal to 2.205 avoirdupois pounds. For other uses, see Weight (disambiguation). ... Gravity redirects here. ... For other uses, see Force (disambiguation). ... This article is about the SI unit of force. ... The unit kilogram-force (kgf, often just kg) or kilopond (kp) is defined as the force exerted by one kilogram of mass in standard Earth gravity. ... The avoirdupois (IPA: ; French IPA: ) system is a system of weights (or, properly, mass) based on a pound of sixteen ounces. ... The pound or pound-mass (abbreviations: lb, , lbm, or sometimes in the United States: #) is a unit of mass (sometimes called weight in everyday parlance) in a number of different systems, including the imperial and US and older English systems. ... This article is about post-1824 Imperial units, please see also English unit, U.S. customary unit or Avoirdupois. ... U.S. customary units, also known in the United States as English units[1] (but see English unit) or standard units, are units of measurement that are currently used in the USA, in some cases alongside units from SI (the International System of Units — the modern metric system). ... The pound-force is a non-SI unit of force or weight (properly abbreviated lbf or lbf). The pound-force is equal to a mass of one pound multiplied by the standard acceleration due to gravity on Earth (which is defined as exactly 9. ...


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 by the initials CIPM) recommended in 2005 that the kilogram be redefined in terms of fundamental constants of nature.[1] The International Committee for Weights and Measures is the English name of the Comité international des poids et mesures (CIPM, sometimes written in English Comité International des Poids et Mesures). ...

Contents

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 one-tenth the acceleration due to Earth’s gravity) when acted upon by a force of one newton (symbol: N). For other uses, see Mass (disambiguation). ... This article is about inertia as it applies to local motion. ... For other uses, see Force (disambiguation). ... Acceleration is the time rate of change of velocity and/or direction, and at any point on a velocity-time graph, it is given by the slope of the tangent to the curve at that point. ... The metre (or meter) per second squared is the SI derived unit of acceleration. ... For other uses, see Newton (disambiguation). ...


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 since such objects naturally hover; they are “weightless.” However, since objects in microgravity still retain their mass, an astronaut must exert one hundred times more force to accelerate a 100-kilogram object at the same rate as a 1-kilogram object. See also Mass versus weight, below. For other uses, see Weight (disambiguation). ... For a less technical and generally accessible introduction to the topic, see Introduction to special relativity. ...


SI multiples

Because SI prefixes may not be concatenated 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.[2] For instance, one-millionth of a kilogram is 1 mg (one milligram), not 1 µkg (one microkilogram). An SI prefix (also known as a metric prefix) is a name or associated symbol that precedes a unit of measure (or its symbol) to form a decimal multiple or submultiple. ... Concatenation is a standard operation in computer programming languages (a subset of formal language theory). ... BIC pen cap, about 1 gram. ...

SI multiples for gram (g)
Submultiples Multiples
Value Symbol Name Value Symbol Name
10–1 g dg decigram 101 g dag decagram
10–2 g cg centigram 102 g hg hectogram
10–3 g mg milligram 103 g kg kilogram
10–6 g µg microgram (mcg) 106 g Mg megagram (tonne)
10–9 g ng nanogram 109 g Gg gigagram
10–12 g pg picogram 1012 g Tg teragram
10–15 g fg femtogram 1015 g Pg petagram
10–18 g ag attogram 1018 g Eg exagram
10–21 g zg zeptogram 1021 g Zg zettagram
10–24 g yg yoctogram 1024 g Yg yottagram
Common prefixes are in bold face.[3]
  • 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.[4] 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 term “tonne,” or “metric ton” is instead used.

This article is about the metric tonne. ... This article or section is in need of attention from an expert on the subject. ... This article is about the metric tonne. ...

History

Early definitions

See also Grave (mass) for more on the history of the kilogram.

On 7 April 1795, the gram was decreed in France to be equal to “the absolute weight of a volume of pure water equal to a cube of one hundredth of a meter, and to the temperature of the melting ice.”[5] The regulation of trade and commerce required a practical reference standard in addition to the definition based on fundamental physical properties. Accordingly, a provisional kilogram standard was made as a single-piece, metallic reference standard one thousand times more massive than the gram. now. ... BIC pen cap, about 1 gram. ...


In addition to this provisional kilogram standard, work was commissioned to precisely determine the mass of a cubic decimeter (now defined as one liter) of water. Although the decreed definition of the kilogram specified water at 0 °C—a highly stable temperature point—the scientists chose to redefine the standard and perform their measurements at the most stable density point: the temperature at which water reaches maximum density, which was measured at the time as 4 °C.[6] 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 earlier that year.[7] Four years later in 1799, an all-platinum prototype kilogram, the Kilogramme des Archive (Kilogram of the Archives), 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 kilogram was defined to be equal to the mass of the Kilogram of the Archives and this standard stood for the next ninety years. The litre or liter (see spelling differences) is a unit of volume. ...


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 right-circular cylinder (height = diameter) of 39.17 mm to minimize its surface area.[8] The IPK and six of its official copies are stored in an environmentally monitored vault 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 40 years. Look up si, Si, SI in Wiktionary, the free dictionary. ... Metrology (from Greek metron (measure), and -logy) is the science of measurement. ... An alloy is a homogeneous hybrid of two or more elements, at least one of which is a metal, and where the resulting material has metallic properties. ... General Name, Symbol, Number platinum, Pt, 78 Chemical series transition metals Group, Period, Block 10, 6, d Appearance grayish white Standard atomic weight 195. ... This article is about the chemical element. ... A millimetre (American spelling: millimeter, symbol mm) is an SI unit of length that is equal to one thousandth of a metre. ... The Bureau International des Poids et Mesures (International Bureau of Weights and Measures, or BIPM) is a standards organization, one of the three organizations established to maintain the SI system under the terms of the Metre Convention. ... Road to Sèvres, Jean-Baptiste Camille Corot, 1855-1865. ... This article is about the capital of France. ...


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 eighty-four years prior, and was formally ratified as the kilogram by the 1st CGPM in 1889.[8] Modern measurements of the density of purified water that has a carefully controlled isotopic composition (known as Vienna Standard Mean Ocean Water) show that a cubic decimeter of water at its point of maximum density, 3.984 °C, has a mass that is 25.05 parts per million less than the kilogram.[9] 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 208 years ago when making their measurements of water’s properties and in manufacturing the Kilogram of the Archives. The General Conference on Weights and Measures is the English name of the Conférence générale des poids et mesures (CGPM, never GCWM). ... VSMOW, or Vienna Standard Mean Ocean Water, is an isotopic water standard defined in 1968 by the International Atomic Energy Agency. ...


Stability of the International Prototype Kilogram

By definition, the error in the measured value of the IPK’s mass is exactly zero; the IPK is the kilogram. However, any changes in the IPK’s mass over time can be deduced by comparing its mass to that of its official copies stored throughout the world, a process called “periodic verification.” For instance, the U.S. owns three kilogram standards, two of which, K4 and K20, are from the original batch of 40 replicas delivered in 1884. The K20 standard was designated as the primary national standard of mass for the U.S. Both of these, as well as those from other nations, are periodically returned to the BIPM for verification.[10]


Note that the masses of the replicas are not precisely equal to that of the IPK; their masses are calibrated and documented as offset values. For instance, K20, the U.S.’s primary standard, originally had an official mass of 1 kg – 39 µg in 1889; that is to say, K20 was 39 µg less than the IPK. A verification performed in 1948 showed a mass of 1 kg – 19 µg. The latest verification performed in 1999 shows a mass identical to its original 1889 value. The mass of K4, the U.S.’s check standard, as of 1999 was officially calibrated as 1 kg – 116 µg. However, it was 41 µg more massive (in comparison to the IPK) in 1889.


Since the IPK and its replicas are stored in air (albeit under two or more nested bell jars), they adsorb atmospheric contamination onto their surfaces and gain mass. Accordingly, they are cleaned in preparation for periodic verifications—a process the BIPM developed between 1939 and 1946 known as “the BIPM cleaning method” that includes steam cleaning, lightly rubbing with chemical-soaked chamois, and allowing the prototypes to settle for 7–10 days. Cleaning the prototypes removes between 5 and 60 µg of contamination depending largely on the time elapsed since the last cleaning. Further, a second cleaning can remove up to 10 µg more. After cleaning—even when they are stored in their bell jars—the IPK and its replicas immediately begin gaining mass again. The BIPM even developed a model of this gain and concluded that it averaged 1.11 µg per month for the first 3 months after cleaning and then decreased to an average of about 1 µg per year thereafter. Since check standards like K4 are not cleaned for routine calibrations of other mass standards—a precaution to minimize the potential for wear and handling damage—the BIPM’s model has been used as an “after cleaning” correction factor. Chamois leather is leather made from the skin of the chamois, although the term is also commonly used to refer to cloths made from the skin of other animals or a synthetic material version. ...


Because the first forty official copies are made of the same alloy as the IPK and are stored under similar conditions, periodic verifications using a large number of replicas—especially the national primary standards, which are rarely used—can convincingly demonstrate the stability of the IPK. What has become clear after the third periodic verification performed between 1988 and 1992 is that the mass of the IPK lost perhaps 50 µg over the last century, and possibly significantly more, in comparison to its official copies.[11][12] The answer as to why this might be the case has proved elusive for physicists who have dedicated their careers to the SI unit of mass. No plausible mechanism has been proposed to explain either a steady decrease in the mass of the IPK, or an increase in that of its replicas dispersed throughout the world.[13] Further, the IPK exhibits an instability of about 30 µg over a period of about a month in its after-cleaned mass.[14] The precise reason for this short-term instability is not understood but is thought to entail surface effects: microscopic differences in their polished surfaces, possibly aggravated by hydrogen absorbtion due to catalysis of VOCs and the hydrocarbon-based solvents used to clean the prototypes.[15] What is known is that the past assumption that the cleaning process reliably restores the prototypes to their original value is false and that the BIPM’s after-cleaning correction factor is useful only for long-term trends. Scientists are seeing far greater variability in the prototypes than previously believed. Further, there is no technical means available to know whether or not the entire worldwide ensemble of prototypes suffer from even greater long-term trends upwards or downwards because their mass “relative to an invariant of nature is unknown at a level below 1000 µg over a period of 100 or even 50 years.”[11] This article is about the chemistry of hydrogen. ... In chemistry and biology, catalysis is the acceleration (increase in rate) of a chemical reaction by means of a substance, called a catalyst, that is itself not consumed by the overall reaction. ... This article describes a highly specialized aspect of its subject in the Terminology and legal definitions section. ... Oil refineries are key to obtaining hydrocarbons; crude oil is processed through several stages to form desirable hydrocarbons, used in fuel and other commercial products. ...


The relative change in mass and the instability in the IPK has prompted research into improved methods to obtain a smooth surface finish using diamond-turning on newly manufactured replicas and has intensified the search for a new definition of the kilogram. See Proposed future definitions, below.[16]


Importance of the kilogram

The kilogram underpins the entire SI system of measurement as it is currently defined and structured, so its stability is crucial. For instance, the newton—the SI unit of force—is defined as the force necessary to accelerate the kilogram by one meter per second². Accordingly, if the mass of the IPK were to change slightly, so too would the newton by a proportional degree so that the acceleration would remain at precisely one meter/second². In turn, the pascal—the SI unit of pressure—is defined in terms of the newton. This chain of dependency follows to most of the electrical units. For instance, the joule, which is the electrical and mechanical unit of energy, is defined as the energy expended when a force of one newton acts through one meter. The ampere is also defined relative to the newton. With the magnitude of two of the primary units of electricity thus determined by the kilogram, so too follow most of the rest; namely, the watt, ohm, coulomb, farad, siemens, henry, and weber. From there, the measurement of light (the candela, lumen, and lux) is in turn affected. For other uses, see Newton (disambiguation). ... For other uses, see Pascal. ... The joule (IPA: or ) (symbol: J) is the SI unit of energy. ... This article is about the unit of length. ... For other uses, see Ampere (disambiguation). ... For other uses, see Watt (disambiguation). ... A multimeter can be used to measure resistance in ohms. ... The coulomb (symbol: C) is the SI unit of electric charge. ... Examples of various types of capacitors. ... The siemens (symbol: S) is the SI derived unit of electric conductance. ... The henry (symbol H) is the SI unit of inductance. ... In physics, the weber (symbol: Wb) is the SI unit of magnetic flux. ... Photopic (black) and scotopic [1] (green) luminosity functions. ... The lumen (symbol: lm) is the SI unit of luminous flux. ... The lux (symbol: lx) is the SI derived unit of illuminance or illumination. ...


Clearly, having the magnitude of many of the units comprising the SI system of measurement ultimately defined by the mass of a 128-year-old piece of metal is a tenuous state of affairs. The quality of the IPK must be diligently protected in order to preserve the integrity of the SI system. 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 1/299,792,458 of a second. However, the meter’s practical realization typically takes the form of a helium-neon laser, and the meter’s length is delineated—not defined—as 1,579,800.298 728 wavelengths of light from this laser. Note that the redefinition of the meter in terms of a duration of one second reduced the uncertainty in the wavelength of the laser light. Now suppose that the official measurement of the second were found to have drifted by a few parts per billion (actually it is exquisitely stable). There would be no automatic effect on many of the SI units of measurement because, as with the meter, the duration of the second is often abstracted through other physical principles underlying their practical realizations. 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 real-world 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 physicists were to definitively prove that the IPK’s value had changed, a simple solution would be to simply redefine the kilogram as being equal to 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 µg.” This article is about the unit of length. ...


The long-term 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 few parts in 108 (~30 µg) required to compete with the IPK. The NIST’s implementation of the watt balance, as of late 2007 was approaching the level where scientists could resolve a difference of about 25 µg. See Proposed future definitions below. The watt balance is an electromechanical apparatus used for the precise measurement of the SI unit of electric current, the ampere. ...


Mass versus weight

The distinction between the two

Matter’s mass strongly influences many familiar kinetic properties.
Matter’s mass strongly influences many familiar kinetic properties.

As stated above in The nature of mass, the kilogram is a unit of mass, which is an inertial property. Inertia is the property that is sensed when pushing horizontally to accelerate a bowling ball that is resting on a level, smooth surface. This is quite distinct from “weight,” which is the downwards gravitational force of the bowling ball that one must counter when holding it off the floor. Unless relativistic effects apply, mass is an unchanging, universal property of matter that is unaffected by gravity. Weight on the other hand, is a property of matter that is entirely dependent upon the local strength of gravity. For instance, an astronaut’s weight on the Moon is one-sixth of that on the Earth, whereas his mass has changed little during the trip. Consequently, wherever the physics of recoil kinetics (mass, velocity, inertia, inelastic and elastic collisions) dominate and the influence of gravity is a negligible factor, the behavior of objects remains consistent even where gravity is relatively weak. For instance, billiard balls on a billiards table would scatter and recoil with the same speeds and energies after a break shot on the Moon as on Earth; they would however, drop into the pockets much more slowly. Image File history File linksMetadata Size of this preview: 800 × 533 pixelsFull resolution (2310 × 1540 pixel, file size: 1. ... Image File history File linksMetadata Size of this preview: 800 × 533 pixelsFull resolution (2310 × 1540 pixel, file size: 1. ... A ten-pin bowling ball and two pins A bowling ball is a round ball made from rubber, urethane, plastic, reactive resin (solid, particle, or pearl) or a combination of these materials which is used in the sport of bowling. ... For a less technical and generally accessible introduction to the topic, see Introduction to special relativity. ... An inelastic collision is a collision in which some of the kinetic energy of the colliding bodies is converted into internal energy in at least one body such that kinetic energy is not conserved. ... As long as black-body radiation (not shown) doesn’t escape a system, atoms in thermal agitation undergo essentially elastic collisions. ...


In the physical sciences, the terms “mass” and “weight” are rigidly defined as separate measures in order to enforce clarity and precision. In everyday use, given that all masses on Earth have weight and this relationship is usually highly proportional,[17] “weight” often serves to describe both properties, its meaning being dependent upon context. For example, in commerce, the “net weight” of retail products actually refers to mass and is properly expressed in pounds (U.S.) or kilograms (see also Pound: Use in commerce). Conversely, the “load index” rating on automobile tires, which specifies the maximum structural load for a tire in kilograms, refers to weight; that is, the force due to gravity. == Headline text ==cant there be some kind of picture somewhere so i can see by picture???? Physical science is a encompassing term for the branches of natural science, and science, that study non-living systems, in contrast to the biological sciences. ... For other uses, see Mass (disambiguation). ... For other uses, see Weight (disambiguation). ... The pound or pound-mass (abbreviations: lb, , lbm, or sometimes in the United States: #) is a unit of mass (sometimes called weight in everyday parlance) in a number of different systems, including the imperial and US and older English systems. ... Automobile tires are described by an alphanumeric code which is generally molded into the side-wall of the tire. ... This article or section does not adequately cite its references or sources. ...


Converting to kilogram-force and newtons

When an object’s weight (its gravitational force) is expressed in kilograms, the unit of measure is not a true kilogram; it is the kilogram-force (kgf or kg-f), also known as the kilopond (kp), which is a non-SI unit of force. All objects on Earth are subject to a gravitational acceleration of approximately 9.8 m/s². The CGPM (also known as the “General Conference on Weights and Measures”) fixed the value of standard gravity at precisely 9.80665 m/s² so that disciplines such as metrology would have a standard value for converting units of defined mass into defined forces and pressures. In fact, the kilogram-force is defined as precisely 9.80665 newtons. As a practical matter, gravitational acceleration (symbol: g) varies slightly with latitude, elevation and subsurface density; these variations are typically only a few tenths of a percent. See also Gravimetry. The unit kilogram-force (kgf, often just kg) or kilopond (kp) is defined as the force exerted by one kilogram of mass in standard Earth gravity. ... The General Conference on Weights and Measures is the English name of the Conférence générale des poids et mesures (CGPM, never GCWM). ... g (also gee, g-force or g-load) is a non-SI unit of acceleration defined as exactly 9. ... Metrology (from Greek metron (measure), and -logy) is the science of measurement. ... This article is about pressure in the physical sciences. ... This article is about the geographical term. ... Elevation histogram of the surface of the Earth – approximately 71% of the Earths surface is covered with water. ... Gravimetry is the measurement of a gravitational field. ...


Professionals in engineering and scientific disciplines involving accelerations and kinetic energies rigorously maintain the distinctions between mass, force, and weight, as well as their respective units of measure. Engineers in disciplines involving weight loading (force on a structure due to gravity), such as structural engineering, first convert loads due to objects like concrete and automobiles—which are always tallied in kilograms—to newtons before continuing with their calculations. Primarily, this is because material properties like elastic modulus are measured and published in terms of the newton and pascal (a unit of pressure derived from the newton). For all practical engineering purposes on Earth, mass in kilograms is converted to weight in newtons by multiplying by 9.80665 (standard gravity). The cars of a roller coaster reach their maximum kinetic energy when at the bottom of their path. ... For other uses, see Weight (disambiguation). ... This article or section does not adequately cite its references or sources. ... Taipei 101, the worlds tallest building as of 2004. ... An elastic modulus, or modulus of elasticity, is the mathematical description of an object or substances tendency to be deformed when a force is applied to it. ... For other uses, see Pascal. ...


Buoyancy and “conventional mass”

Regardless of the fluid in which an object is immersed (gas or liquid), the buoyancy of an object is proportional to the mass of the fluid it displaces.
Regardless of the fluid in which an object is immersed (gas or liquid), the buoyancy of an object is proportional to the mass of the fluid it displaces.

The masses of objects are relatively invariant whereas their weights vary slightly with changes in barometric pressure, such as with changes in weather and altitude. This is because objects have volume and therefore have a buoyant effect in air. Buoyancy—a force that opposes gravity—reduces the weight of all objects immersed in fluids. This means that objects with precisely the same mass but with different densities displace different volumes and therefore have different buoyancies and weights. Image File history File links Submerged-and-Displacing. ... Image File history File links Submerged-and-Displacing. ... For other uses, see Volume (disambiguation). ... In physics, buoyancy is the upward force on an object produced by the surrounding fluid (i. ... A fluid is defined as a substance that continually deforms (flows) under an applied shear stress regardless of the magnitude of the applied stress. ... For other uses, see Density (disambiguation). ...


Normally, the effect of air buoyancy is too small to be of any consequence in normal day-to-day activities. For instance, buoyancy’s diminishing effect upon one’s body weight (a relatively low-density object) is 1/860 that of gravity and variations in barometric pressure rarely affect one’s weight more than ±1 part in 30,000.[18] In metrology however, mass standards are calibrated with extreme accuracy, so air density must be taken into account to allow for buoyancy effects. Metrology (from Greek metron (measure), and -logy) is the science of measurement. ...


Given the extremely high cost of platinum-iridium prototypes, high-quality “working” standards are made of special stainless steel alloys, which occupy greater volume than those made of platinum-iridium. For convenience, a standard value of buoyancy relative to stainless steel was developed for metrology work and this results in the term “conventional mass.”[19] Conventional mass is defined as follows: “For a mass at 20 °C, ‘conventional mass’ is the mass of a reference standard of density 8000 kg/m³ which it balances in air with a density of 1.2 kg/m³.” The effect is a small one, 150 ppm for stainless steel mass standards, but the appropriate corrections are made during the calibration of all precision mass standards so that they have the true mass indicated on them. In routine laboratory use however, the reading on a precision scale when a stainless steel standard is placed upon it is actually its conventional mass; that is, its true mass minus buoyancy. Also, any object compared to a stainless steel mass standard has its conventional mass measured; that is, its true mass minus some (usually unknown) degree of buoyancy. General Name, Symbol, Number platinum, Pt, 78 Chemical series transition metals Group, Period, Block 10, 6, d Appearance grayish white Standard atomic weight 195. ... This article is about the chemical element. ... The 630 foot (192 m) high, stainless-clad (type 304) Gateway Arch defines St. ... The parts-per notations are used to denote low concentrations of chemical elements. ...


Types of scales and what they measure

A balance-type weighing scale: Unaffected by the strength of gravity
A balance-type weighing scale: Unaffected by the strength of gravity
Load-cell based bathroom scale: Affected by the strength of gravity
Load-cell based bathroom scale: Affected by the strength of gravity

Technically, whenever someone stands on a balance-beam-type scale at a doctor’s office, they are truly having their mass measured. This is because balances (“dual-pan” mass comparators) compare the weight of the mass on the platform with that of the sliding counterweights on the beams; gravity serves only as the force-generating mechanism that allows the needle to diverge from the “balanced” (null) point. Balances can be used on the Moon with no change in the reading. Conversely, whenever someone steps onto spring-based or digital load cell-based scales (single-pan devices), they are technically having their weight (force due to strength of gravity) measured. On force-measuring instruments such as these, variations in the strength of gravity affect the reading. As a practical matter, when force-measuring scales are used in commerce or hospitals, they are calibrated on-site and certified on that basis so the measure is mass, expressed in kilograms, to the desired level of accuracy.[20] Image File history File links Size of this preview: 800 × 574 pixel Image in higher resolution (1932 × 1387 pixel, file size: 198 KB, MIME type: image/jpeg) Photo of a four-beam mechanical balance scale. ... Image File history File links Size of this preview: 800 × 574 pixel Image in higher resolution (1932 × 1387 pixel, file size: 198 KB, MIME type: image/jpeg) Photo of a four-beam mechanical balance scale. ... Image File history File linksMetadata No higher resolution available. ... Image File history File linksMetadata No higher resolution available. ... Digital kitchen scales. ... Digital kitchen scales. ... A single-point load cell A load cell is typically an electronic device (transducer) that is used to convert a force into an electrical signal. ...


Proposed future definitions

In the following section, wherever numeric equalities are shown in ‘concise form’—such as 1.854 87(14) × 1043—the two digits between the parentheses denotes the uncertainty (the standard deviation at 68.27% confidence level) in the two least significant digits of the mantissa.

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 platinum-iridium 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 upon equipment and procedures that enable the reproducible production of new, kilogram-mass 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 essentially devices that measure either the acceleration or weight of hand-tuned, 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 but one, the Avogadro Project, would precisely fix one or more constants of nature at a defined value. These different approaches are as follows: “Uncertain” redirects here. ... In probability and statistics, the standard deviation of a probability distribution, random variable, or population or multiset of values is a measure of the spread of its values. ... This article is about the unit of length. ...


Atom-counting approaches

Avogadro project
A single crystal of silicon known as a boule.
A single crystal of silicon known as a boule.

An Avogadro constant-based approach, known as the Avogadro project, attempts to define and delineate the kilogram as a quantity of silicon atoms. Silicon was chosen because a commercial infrastructure with mature processes for creating defect-free, ultra-pure monocrystalline silicon already exists to service the semiconductor industry. To make a practical realization of the kilogram, a silicon boule (a rod-like, single-crystal ingot) would be produced. Its isotopic composition would be measured with a mass spectrometer to determine its average atomic mass. The rod would be cut, ground, and polished into spheres. The size of a select sphere would be measured using optical interferometry. The precise lattice spacing between the atoms in its crystal structure (≈192 pm) would be measured using a scanning X-ray interferometer. Amazingly, this permits its atomic spacing to be determined with an uncertainty of only three parts in a billion. With the size of the sphere, its average atomic mass, and the atomic spacing known, the required number of atoms in the sphere could be calculated with sufficient precision and uncertainty to enable it to be ground down to the desired quantity of atoms (mass). Image File history File links Monokristalines_Silizium_für_die_Waferherstellung. ... Image File history File links Monokristalines_Silizium_für_die_Waferherstellung. ... Silicon boule for the production of wafers. ... The Avogadro constant (symbols: L, NA), also called the Avogadro number and, in German scientific literature, sometimes also known as the Loschmidt constant/number, is formally defined to be the number of entities in one mole,[1][2] that is the number of carbon-12 atoms in 12 grams (0. ... Not to be confused with Silicone. ... A semiconductor is a solid whose electrical conductivity is in between that of a conductor and that of an insulator, and can be controlled over a wide range, either permanently or dynamically. ... Silicon boule for the production of wafers. ... For other uses, see Isotope (disambiguation). ... Mass spectrometry (previously called mass spectroscopy (deprecated)[1] or informally, mass-spec and MS) is an analytical technique used to measure the mass-to-charge ratio of ions. ... It has been suggested that Optical interferometry be merged into this article or section. ...


Experiments are planned for the Avogadro Project’s silicon sphere to determine whether its mass is most stable when stored in a vacuum, a partial vacuum, or ambient pressure. However, no technical means currently exist to prove a stability any better than that of the IPK’s because the most sensitive and accurate measurements of mass are made with balances like the BIPM’s FB-2 flexure-strip balance (see Links to photographs, below). Balances can only compare the mass of a silicon sphere to that of a reference mass. Given the latest understanding of the lack of relative stability between the IPK and its replicas, there is no known, perfectly stable mass artifact to compare against. Scales capable of measuring mass relative to an invariant of nature at the 30-parts-per-billion level of uncertainty of the IPK do not yet exist. Another issue to be overcome is that silicon oxidizes and forms a thin layer of silicon dioxide (common glass). This layer slightly increases the mass of the sphere and its effect must be accounted for when grinding the sphere to its finish dimension. Oxidation is not an issue with platinum and iridium, both of which are noble metals that are roughly as cathodic as oxygen and therefore don’t oxidize unless coaxed to do so in the laboratory. The presence of a thin glass layer on a silicon-sphere mass prototype places additional restrictions on the procedures that might be suitable to clean it. R-phrases R42 R43 R49 S-phrases S22 S36 S37 S45 S53 Flash point non-flammable Supplementary data page Structure and properties n, εr, etc. ... Noble metals are metals that are resistant to corrosion or oxidation, unlike most base metals. ... The galvanic series determines the nobility of metals and semi-metals. ...


Carbon-12

Though not offering a practical realization, this definition would precisely define the magnitude of the kilogram in terms of a certain number of carbon-12 atoms. The mole is currently defined as “the quantity of ‘entities’ (elementary particles like atoms or molecules) as there are atoms in 12 grams of carbon-12.” Thus, the current definition of the mole requires that 1000/12 (83⅓) moles of C-12 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 6.022 141 79(30) × 1023 atoms (2006 CODATA value). This new definition of the kilogram proposes to fix the Avogadro constant at precisely 6.022 141 79 × 1023 and the kilogram would be defined as “the mass equal to that of 83⅓ × 6.022 141 79 × 1023 atoms of carbon-12.” Carbon 12 is a stable isotope of the element carbon. ... The mole (symbol: mol) is the SI base unit that measures an amount of substance. ... The Avogadro constant (symbols: L, NA), also called the Avogadro number and, in German scientific literature, sometimes also known as the Loschmidt constant/number, is formally defined to be the number of entities in one mole,[1][2] that is the number of carbon-12 atoms in 12 grams (0. ...


Currently, the uncertainty in the Avogadro constant is determined by the uncertainty in the measured mass of carbon-12 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 carbon-12 atoms become available. Electronic realizations of the kilogram would be recalibrated as required. In an electronic definition of the kilogram, the mole would remain decoupled from the kilogram and could continue to be defined in terms of 12 grams of carbon-12. In that case, 83⅓ moles of carbon-12 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 carbon-12-based definition proposes to define the Avogadro constant as being precisely 84,446,8863 (≅6.022 140 98 × 1023) atoms. An imaginary realization of a 12-gram mass prototype would be a cube of carbon-12 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 carbon-12.” 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 carbon-12: 50,184,508,190,229,061,679,538 atoms.[21]


Ion accumulation

Another Avogadro-based approach, ion accumulation, would define and delineate the kilogram by creating metal mass artifacts. It would do so by accumulating of gold or bismuth ions (atoms stripped of an electron) and counting them by measuring the electrical current required to neutralize the ions. Gold and bismuth are used because, unlike most other elements, they each have only one naturally occurring isotope. This article is about the electrically charged particle. ... GOLD refers to one of the following: GOLD (IEEE) is an IEEE program designed to garner more student members at the university level (Graduates of the Last Decade). ... General Name, Symbol, Number bismuth, Bi, 83 Chemical series poor metals Group, Period, Block 15, 6, p Appearance lustrous pink Standard atomic weight 208. ... This article is about the electrically charged particle. ... For other uses, see Isotope (disambiguation). ...


With a gold-based definition of the kilogram for instance, the molecular weight of gold would be fixed as precisely 196.966 569 g (from the current value of 196.966 569(4) grams). As with a definition based upon carbon-12, the Avogadro constant would also be fixed. The kilogram would then be defined as “the mass equal to that of precisely 1000/196.966 569 × 6.022 141 79 × 1023 atoms of gold” (≅5.077 003 7021 moles of gold atoms).


Ion-accumulation techniques, while a relatively new field of study, have advanced rapidly. In 2003, experiments with gold at a current of only 10 µa demonstrated a relative uncertainty of 1.5%. Yet, follow-on experiments using bismuth ions and a current of 30 mA were expected to accumulate a mass of 30 g in six days and to have a relative uncertainty of better than 1 part in 106.[22]


The difficulty with ion-accumulation-based standards is in obtaining truly practical mass artifacts. Gold, while dense and a noble metal (resistant to oxidation and the formation of other compounds), is extremely soft so mass artifacts would require extraordinary care to avoid wear. Bismuth, while an inexpensive metal for experiments, would not produce stable artifacts because it readily oxidizes and forms other chemical compounds. Iridium and platinum are composed of two and six isotopes respectively and this places an upper limit on relative uncertainty. Noble metals are metals that are resistant to corrosion or oxidation, unlike most base metals. ...


Electronic approaches

Watt balance
The NIST’s watt balance is a project of the U.S. Government to develop an “electronic kilogram.” The vacuum chamber dome, which lowers over the entire apparatus, is visible at top.

The watt balance is essentially an ampere balance with an extra calibration step that nulls the effect of geometry. The electrical current in the watt balance is delineated by a Josephson voltage standard, which allows voltages to be linked to an invariant constant of nature with extremely high precision and stability, and the circuit resistance is calibrated against a quantum Hall resistance standard. The watt balance requires exquisitely precise measurement of gravity in a laboratory (see “FG-5 absolute gravimeter” in Links to photographs, below) and compares this acceleration to the electrical power necessary to counter it. For instance, the gravity gradient of 3.1 µGal/cm (3 parts in 109) is accounted for when the elevation of the center of the gravimeter differs from that of the nearby test mass. As of late 2007, the NIST’s implementation of the watt balance was approaching the level where scientists could resolve a difference of about 25 µg and the U.K.’s National Physical Laboratory’s watt balance was demonstrating an uncertainty of 70 µg.[23] Image File history File links Metadata No higher resolution available. ... Image File history File links Metadata No higher resolution available. ... NIST logo The National Institute of Standards and Technology (NIST, formerly known as The National Bureau of Standards) is a non-regulatory agency of the United States Department of Commerce’s Technology Administration. ... The watt balance is an electromechanical apparatus used for the precise measurement of the SI unit of electric current, the ampere. ... The ampere balance (also current balance or Kelvin balance) is an electromechanical apparatus used for the precise measurement of the SI unit of electric current, the ampere. ... The Josephson effect is the phenomenon of current flow across two weakly coupled superconductors, separated by a very thin insulating barrier. ... Hall effect diagram, showing electron flow (rather than conventional current). ... The gal or galileo is the CGS unit of acceleration. ... An accelerometer or gravimeter is a device for measuring acceleration and the effects of gravity. ... NIST logo The National Institute of Standards and Technology (NIST, formerly known as The National Bureau of Standards) is a non-regulatory agency of the United States Department of Commerce’s Technology Administration. ... The National Physical Laboratory (NPL) is the national measurement standards laboratory for the United Kingdom, based at Bushy Park in Teddington in the London Borough of Richmond upon Thames. ...


Ultimately, the watt balance would define the kilogram in terms of the Planck constant, which is a measure that relates the energy of photons to their frequency. The Planck constant would be fixed, where h = 6.626 068 96 × 10–34 J·s (from the 2006 CODATA value of 6.626 068 96(33) × 10–34 J·s) and the kilogram would be defined as “the mass of a body at rest whose equivalent energy equals the energy of photons whose frequencies sum to 1.356 392 733 × 1050 Hz.”[24] A commemoration plaque for Max Planck on his discovery of Plancks constant, in front of Humboldt University, Berlin. ... The joule (IPA: or ) (symbol: J) is the SI unit of energy. ... This article is about the unit of time. ... CODATA (Committee on Data for Science and Technology) was established in 1966 as an interdisciplinary committee of the International Council of Science (ICSU), formerly the International Council of Scientific Unions. ...


The virtue of electronic-based realizations like the watt balance is that the definition and dissemination of the kilogram would no longer be dependent upon the stability of kilogram prototypes, which must be very carefully handled and stored. It would free physicists from the need to rely on assumptions about the stability of those prototypes, including those that would be manufactured under atom-counting schemes. Instead, hand-tuned, close-approximation mass standards would simply be weighed and documented as being equal to one kilogram plus an offset value. With scales, the kilogram would not only be defined in electrical terms, it would also be delineated in electrical terms. Mass artifacts calibrated in a watt balance would effectively become transfer standards. Further, one additional term in all scale-based realizations—acceleration due to gravity—is currently measured using dropping-mass absolute gravimeters that contain an iodine-stabilized HeNe laser interferometer. The fringe-signal, frequency-sweep output from the interferometer is measured with a rubidium atomic clock. Thus, the “gravity” term in the delineation of an all-electronic kilogram would also be measured relative to invariants of nature. A helium-neon laser, usually called a HeNe laser, is a type of small gas laser. ... It has been suggested that Optical interferometry be merged into this article or section. ... “Nuclear Clock” redirects here. ...


Scales also permit more flexibility in choosing materials with especially desirable properties for mass standards. For instance, 90Pt–Ir could continue to be used so that the specific gravity of newly produced mass standards would be the same as existing national primary and check standards. This would reduce the relative uncertainty when making mass comparisons in air. Alternately, entirely different materials and constructions could be explored with the objective of producing mass standards with greater stability. For instance, osmium-iridium alloys could be investigated if platinum’s propensity to absorb hydrogen (due to catalysis of VOCs and hydrocarbon-based cleaning solvents) and atmospheric mercury proved to be sources of instability. Also, vapor-deposited, protective ceramic coatings like nitrides could be investigated for their suitability to isolate these new alloys. General Name, Symbol, Number osmium, Os, 76 Chemical series transition metals Group, Period, Block 8, 6, d Appearance silvery, blue cast Standard atomic weight 190. ... General Name, Symbol, Number mercury, Hg, 80 Chemical series transition metals Group, Period, Block 12, 6, d Appearance silvery Standard atomic weight 200. ... Definition The nitride ion is very very gay and retarded A nitride (compound) is a compound that has nitrogen with more electropositive elements. ...


Ampere-based force

This approach would define the kilogram as “the mass which would be accelerated at precisely 2 × 10–7 m/s² when subjected to the per-meter 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 1/1.602 176 487 × 10–19 (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 1.602 176 487 × 10–19 coulomb (from the current 2006 CODATA value of 1.602 176 487(40) × 10–19), 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. For other uses, see Ampere (disambiguation). ... The elementary charge (symbol e or sometimes q) is the electric charge carried by a single proton, or equivalently, the negative of the electric charge carried by a single electron. ... The coulomb (symbol: C) is the SI unit of electric charge. ...


The virtue of a practical realization based upon this definition is that unlike the watt balance and other scale-based 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, 30-gram mass supported by diamagnetic levitation never achieved an uncertainty better than 10 parts in 106. Magnetic hysteresis was one of the limiting issues. Other groups are continuing this line of research using different techniques to levitate the mass.[25] A magnet levitating above a high-temperature superconductor, cooled with liquid nitrogen. ... A system with hysteresis exhibits path-dependence, or rate-independent memory. Consider a deterministic system with no hysteresis and no dynamics. ...


See also

This article is about inertia as it applies to local motion. ... “SI” redirects here. ... The International Bureau of Weights and Measures is the English name of the Bureau international des poids et mesures (BIPM, often written in English Bureau International des Poids et Mesures), a standards organisation, one of the three organizations established to maintain the International System of Units (SI) under the terms... The International Committee for Weights and Measures is the English name of the Comité international des poids et mesures (CIPM, sometimes written in English Comité International des Poids et Mesures). ... The General Conference on Weights and Measures is the English name of the Conférence générale des poids et mesures (CGPM, never GCWM). ... BIC pen cap, about 1 gram. ... now. ... Gravimetry is the measurement of a gravitational field. ... The unit kilogram-force (kgf, often just kg) or kilopond (kp) is defined as the force exerted by one kilogram of mass in standard Earth gravity. ... The International System of Units (symbol: SI) (for the French phrase Syst me International dUnit s) is the most widely used system of units. ... For other uses, see Mass (disambiguation). ... NIST logo The National Institute of Standards and Technology (NIST, formerly known as The National Bureau of Standards) is a non-regulatory agency of the United States Department of Commerce’s Technology Administration. ... For other uses, see Newton (disambiguation). ... The SI system of units defines seven SI base units: physical units defined by an operational definition. ... g (also gee, g-force or g-load) is a non-SI unit of acceleration defined as exactly 9. ... This article is about the metric tonne. ... The watt balance is an electromechanical apparatus used for the precise measurement of the SI unit of electric current, the ampere. ... For other uses, see Weight (disambiguation). ...

Notes

  1. ^ Proceedings of the 94th meeting (October 2005) of the International Committee for Weights and Measures, (1.1 MB zip file, here)
  2. ^ NIST: SI prefixes (link to Web site).
  3. ^ Criterion: A combined total of at least 250,000 Google hits on both the U.S. spelling (-gram) and the U.K./International spelling (-gramme).
  4. ^ The practice of using the abbreviation “mcg” rather than the SI symbol “µg” was formally mandated for medical practitioners in 2004 by the Joint Commission on Accreditation of Healthcare Organizations (JCAHO) in their “Do Not Use” List: Abbreviations, Acronyms, and Symbols because hand-writen expressions of “µg” can be confused with “mg”, resulting in a thousand-fold overdosing. The mandate was also adopted by the Institute for Safe Medication Practices.
  5. ^ Decree relating to the weights and measurements
  6. ^ Citation: L'Histoire Du Mètre, La Détermination De L'Unité De Poids, link to Web site here.
  7. ^ Citation: History of the kilogram
  8. ^ a b New Techniques in the Manufacture of Platinum-Iridium Mass Standards, T. J. Quinn, Platinum Metals Rev., 1986, 30, (2), pp. 74–79
  9. ^ Water Structure and Science, Water Properties, Density maximum (and molar volume) at temperature of maximum density, a (by London South Bank University). Link to Web site.
  10. ^ Extraordinary care is exercised when transporting prototypes. In 1984, the K4 and K20 standards were hand-carried in the passenger section of a commercial airliner.
  11. ^ a b Redefinition of the kilogram: a decision whose time has come, Ian M. Mills et al., Metrologia 42 (2005), 71–80
  12. ^ The Third Periodic Verification of National Prototypes of the Kilogram (1988–1992), G. Girard, Metrologia 31 (1994) 317–336
  13. ^ The SI unit of mass, Richard Davis, Metrologia 40 (2003), 299–305. Note that if the ∆50 µg between the IPK and its replicas was entirely due to wear, the IPK would have to have lost 150 million billion more platinum and iridium atoms over the last century than its replicas. That there would be this much wear, much less a difference of this magnitude, is thought unlikely; 50 µg is roughly the mass of a fingerprint. Many theories have been advanced to explain the data, including one that begins with the observation that the IPK is uniquely stored under three bell jars whereas its six sister replicas stored in the vault with it and the other replicas dispersed throughout the world are stored under only two. This theory is founded on two other facts: that platinum has a strong affinity for mercury, and that atmospheric mercury is significantly more abundant in the atmosphere today than at the time the IPK and its replicas were manufactured. This theory posits that the relative change in mass between the IPK and its replicas is not one of loss at all, and is instead a simple matter that the IPK has gained less than the replicas. This theory is just one of many advanced by the specialists to account for the relative change in mass. To date, each theory has either proven implausible, or there is insufficient data or technical means to either prove or disprove it. Citation: Conjecture why the IPK drifts, R. Steiner, NIST, 11 Sept. 2007.
  14. ^ Report to the CGPM, 14th meeting of the Consultative Committee for Units (CCU), April 2001, 2. (ii); General Conference on Weights and Measures, 22nd Meeting, October 2003, which stated “The kilogram is in need of a new definition because the mass of the prototype is known to vary by several parts in 108 over periods of time of the order of a month…” (3.2 MB ZIP file, here).
  15. ^ Citations: Conjecture why the IPK drifts, R. Steiner, NIST, 11 Sept. 2007; and the BBC’s Getting the measure of a kilogram
  16. ^ General section citations: Recalibration of the U.S. National Prototype Kilogram, R. S. Davis, Journal of Research of the National Bureau of Standards, 90, No. 4, July–August 1985 (5.5 MB PDF here); and The Kilogram and Measurements of Mass and Force, Z. J. Jabbour et al., J. Res. Natl. Inst. Stand. Technol. 106, 2001, 25–46 (3.5 MB PDF, here)
  17. ^ On Earth, masses with densities less than that of air float and have negative weight; that is, they are buoyant. Such masses have positive weight in a vacuum.
  18. ^ Assumptions: An air density of 1160 g/m³, an average density of a human body (with collapsed lungs) equal to that of water, and variations in barometric pressure rarely exceeding ±22 torr. Assumptions primary variables: An altitude of 194 meters above mean sea level (the worldwide median altitude of human habitation), an indoor temperature of 23 °C, a dewpoint of 9 °C, and 760 mmHg sea level–corrected barometric pressure.
  19. ^ International Recommendation OIML R33, International Organization of Legal Metrology.
  20. ^ National General Conference on Weights and Measures, Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices, NIST Handbook 44
  21. ^ Georgia Tech, 21 Sept. 2007 press release: A Better Definition for the Kilogram? Note that the uncertainty in the Avogadro constant narrowed since this proposal was first submitted to American Scientist for publication. The 2006 CODATA value for the Avogadro constant has a relative standard uncertainty of 50 parts per billion and the only cube root values within this uncertainty must fall within the range of 84,446,889.8 ±1.4; that is, there are only three integer cube roots (…89, …90, and …91) in this range and the value 84,446,886 falls outside of it. Unfortunately, none of the three integer values within the range posses the property of their cubes being divisible by twelve; one gram of C-12 could not comprise an integer number of atoms. If the value 84,446,886 was adopted to define the kilogram, many other constants of nature and electrical units would have to be revised an average of about 0.13 part per million. The straightforward adjustment to this approach advanced by the group would instead define the kilogram as “the mass equal to 84,446,8903 × 83⅓ atoms of carbon-12.” This proposed value for the Avogadro constant falls neatly within the measured value (≅6.022 141 84 × 1023 vs. the 2006 CODATA value of 6.022 141 79(30) × 1023) and the proposed definition of the kilogram produces an integer number of atoms in 12 grams of carbon-12, but not for 1 gram nor 1 kilogram.
  22. ^ General Conference on Weights and Measures, 22nd Meeting, October 2003 (3.2 MB ZIP file, here).
  23. ^ NPL: NPL Watt Balance
  24. ^ Hysteresis and Related Error Mechanisms in the NIST Watt Balance Experiment, Joshua P. Schwarz et al., Journal of Research of the National Bureau of Standards and Technology, 106, No. 4, July–August 2001 (888 KB PDF here); and On the redefinition of the kilogram, B. N. Taylor et al., Metrologia 36 (1999), 63–64; and the NIST’s Fundamental Physical Constants: “Energy Equivalents” calculator.
  25. ^ NIST, Beyond the Kilogram: Redefining the International System of Units; and A Watt Balance On Its Side, R. Steiner, NIST, 24 Sept. 2007.

In physics, buoyancy is the upward force on an object produced by the surrounding fluid (i. ... The International Organization of Legal Metrology or Organization Internationale de Métrologie Légale (OIML) is an intergovernmental treaty organization. ... American Scientist (ISSN 0003-0996) is an illustrated bimonthly magazine about science and technology. ...

External links

Links to photographs

The gal or galileo is the CGS unit of acceleration. ...

Glossary

  • Artifact: A human-made object used as a comparative standard in the measurement of a physical quantity.
  • Check standard:
  1. A standard body’s backup replica of the IPK.
  2. A secondary kilogram mass standard used for 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.
  • 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 for delineating the magnitude of a unit of measure.
  • Primary national standard:
  1. A replica of the IPK possessed by a nation
  2. The least used replica of the IPK when a nation possesses more than one.
  • Prototype:
  1. A human-made object that serves as the defining comparative standard in the measurement of a physical quantity.
  2. A human-made object that serves as the comparative standard in the measurement of a physical quantity.
  3. The IPK and any of its replicas
  • Replica: An official copy of the IPK.
  • Transfer standard: An artifact or apparatus that reproduces the magnitude of a unit of measure in a different, usually more practical, form.

  Results from FactBites:
 
Kilogram - definition of Kilogram in Encyclopedia (952 words)
A gram is defined as one thousandth of a kilogram.
The kilogram was originally defined as the mass of one litre of pure water at a temperature of 4 degrees Celsius and standard atmospheric pressure.
The Watt balance uses the current balance that formerly was 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.
Kilogram - Wikipedia, the free encyclopedia (1577 words)
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.
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).
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.
  More results at FactBites »

 
 

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