On 7 April , the gramme , upon which the kilogram is based, was decreed to be equal to "the absolute weight of a volume of pure water equal to a cube of one hundredth of a metre, and at the temperature of the melting ice". On 7 April the metric system was formally defined in French law. Decimal multiples of these units were defined by Greek prefixes : " myria- " 10, , " kilo- " , " hecto- " and " deka- " 10 and submultiples were defined by the Latin prefixes " deci- " 0. The draft definitions enabled provisional copies of the kilograms and metres to be constructed.
The task eventually took more than six years — with delays caused not only by unforeseen technical difficulties but also by the convulsed period of the aftermath of the Revolution. The project was split into two parts — the northern section of In an operation taking six weeks, the baseline was accurately measured using four platinum rods, each of length two toise about 3. After the two surveyors met, each computed the other's baseline in order to cross-check their results and they then recomputed the metre as The final value of the metre was defined in as the computed value from the survey.
In June , platinum prototypes were fabricated according to the measured quantities, the metre des archives defined to be a length of In December of that year, the metric system based on them became by law the sole system of weights and measures in France from until Despite the law, the populace continued to use the old measures. In , Napoleon revoked the law and issued one called the mesures usuelles , restoring the names and quantities of the customary measures but redefined as round multiples of the metric units, so it was a kind of hybrid system.
In , after the collapse of the Napoleonic Empire, the new Assembly reimposed the metric system defined by the laws of and , to take effect in The metrication of France took until about to be completed. Some of the old unit names, especially the livre , originally a unit of mass derived from the Roman libra as was the English pound , but now meaning grams, are still in use today. At the start of the nineteenth century, the French Academy of Sciences' artefacts for length and mass were the only nascent units of the metric system that were defined in terms of formal standards.
Other units based on them, except the litre proved to be short-lived. Pendulum clocks that could keep time in seconds had been in use for about years, but their geometries were local to both latitude and altitude, so there was no standard of timekeeping. Nor had a unit of time been recognised as an essential base unit for the derivation of things like force and acceleration. Some quantities of electricity like charge and potential had been identified, but names and interrelationships of units were not yet established.
A model of interrelated units was first proposed in by the British Association for the Advancement of Science BAAS based on what came to be called the "mechanical" units length, mass and time. Over the following decades, this foundation enabled mechanical , electrical and thermal [ when? In German mathematician Carl-Friedrich Gauss made the first absolute measurements of the Earth's magnetic field using a decimal system based on the use of the millimetre, milligram, and second as the base unit of time.
In the second report  they introduced the concept of a coherent system of units whereby units of length, mass and time were identified as "fundamental units" now known as base units. All other units of measure could be derived hence derived units from these base units. The metre, gram and second were chosen as base units. This was supported by Thomson Lord Kelvin  The concept of naming units of measure after noteworthy scientists was subsequently used for other units.
In , another committee of the BAAS that also counted Maxwell and Thomson among its members and tasked with "the Selection and Nomenclature of Dynamical and Electrical Units" recommended using the cgs system of units. The committee also recommended the names of " dyne " and " erg " for the cgs units of force and energy. The reports recognised two Centimetre-gram-second based systems for electrical units, the Electromagnetic or absolute system of units EMU and the Electrostatic system of units ESU.
In the s Georg Ohm formulated Ohms Law which can be extended to relate power to current, electric potential voltage and resistance. The electrical units of measure did not easily fit into the coherent system of mechanical units defined by the BAAS. Maxwell and Boltzmann had produced theories describing the inter-relational of temperature, pressure and volume of a gas on a microscopic scale but otherwise, in , there was no understanding of the microscopic nature of temperature.
By the end of the nineteenth century, the fundamental macroscopic laws of thermodynamics had been formulated and although techniques existed to measure temperature using empirical techniques, the scientific understanding [ clarification needed ] of the nature of temperature was minimal. The international prototype would also be a "line standard", that is the metre was defined as the distance between two lines marked on the bar, so avoiding the wear problems of end standards.
The French government gave practical support to the creation of an International Metre Commission, which met in Paris in and again in with the participation of about thirty countries. The prototype had a special X-shaped Tresca cross section to minimise the effects of torsional strain during length comparisons. The London firm Johnson Matthey delivered 30 prototype metres and 40 prototype kilograms. X were accepted as the international prototypes. The remainder were either kept as BIPM working copies or distributed to member states as national prototypes.
Following the Convention of the Metre, in the BIPM had custody of two artefacts — one to define length and the other to define mass. Other units of measure which did not rely on specific artefacts were controlled by other bodies. Although the definition of the kilogram remained unchanged throughout the twentieth century, the 3rd CGPM in clarified that the kilogram was a unit of mass , not of weight. The original batch of 40 prototypes adopted in were supplemented from time to time with further prototypes for use by new signatories to the Metre Convention.
The twentieth century history of measurement is marked by five periods: the definition of the coherent MKS system; the intervening 50 years of coexistence of the MKS, cgs and common systems of measures; the Practical system of units prototype of the SI; the introduction of the SI in ; and the evolution of the SI in the latter half century.
The need for an orthogonal electromagnetic unit to resolve the difficulties of defining such units in terms of length, mass and time was identified by Giorgi in As the industrial era dawned in Britain as well as continental Europe and overseas, the cgs system of units as adopted by the British Association for the Advancement of Science in with its plethora of electrical units continued to be the dominant system of measurement, and would remain so for at least the next 60 years.
The advantages were several: it had a variegated set of derived units which, while not quite coherent, were at least homologous; the MKS system lacked a defined unit of electromagnetism at all; the MKS units were inconveniently large for the sciences; customary systems of measures held sway in the United States, Britain and the British empire, and even to some extent in France the birthplace of the metric system, which inhibited adoption of any competing system. Finally, war, nationalism and other political forces inhibited development of the science favouring a coherent system of units. At the 8th CGPM in the need to replace the "International" electrical units with "absolute" units was raised.
The IEC proposal that Giorgi's 'system', denoted informally as MKSX, be adopted was accepted, but no decision was made as to which electrical unit should be the fourth base unit. In J E Sears  [ citation needed ] , proposed that this should be the ampere, but World War II prevented this being formalised until The first and only follow-up comparison of the national standards with the international prototype metre was carried out between and ,   and indicated that the definition of the metre was preserved to within 0.
In response to formal requests made by the International Union of Pure and Applied Physics and by the French government to establish a practical system of units of measure, the CGPM requested the CIPM to prepare recommendations for a single practical system of units of measurement, suitable for adoption by all countries adhering to the Metre Convention. In accordance with astronomical observations, the second was set as a fraction of the year The electromagnetic base unit as required by Giorgi was accepted as the ampere.
Until the advent of the atomic clock , the most reliable timekeeper available to mankind was the earth's rotation. It was natural therefore that the astronomers under the auspice of the International Astronomical Union IAU took the lead in maintaining the standards relating to time.
During the twentieth century it became apparent that the earth's rotation was slowing down resulting in days becoming 1. In accordance with Giorgi's proposals of , the CIPM also recommended that the ampere be the base unit from which electromechanical units would be derived.
The definitions for the ohm and volt that had previously been in use were discarded and these units became derived units based on the ampere. In the CIPM formally adopted a definition of the ampere based on the original EMU definition and redefined the ohm in terms of other base units.
A series of lookup tables defined temperature in terms of inter-related empirical measurements made using various devices. In , definitions relating to temperature had to be clarified.
The Celsius , as an angular measure, was adopted for general use in a number of countries, so in the General Conference on Weights and Measures CGPM recommended that the degree Celsius, as used for the measurement of temperature, be renamed the degree Celsius. At the 9th CGPM, the Celsius temperature scale was renamed the Celsius scale and the scale itself was fixed by defining the triple point of water as 0. This standard, the candela cd which was defined as "the brightness of the full radiator at the temperature of solidification of platinum is 60 new candles per square centimetre ".
The newly accepted definition of the ampere allowed practical and useful coherent definitions of a set of electromagnetic derived units including farad, henry, watt, tesla, weber, volt, ohm, and coulomb. Two derived units, lux and lumen were based on the new candela, and one, degree Celsius, equivalent to the degree Kelvin. Five other miscellaneous derived units completed the draft proposal: radian, steradian, hertz, joule and newton.
In the CIPM proposed the use of wavelength of a specific light source as the standard for defining length, and in the CGPM accepted this proposal using radiation corresponding to a transition between specified energy levels of the krypton 86 atom as the new standard for the metre. The standard metre artefact was retired.
The evolution of the SI after its publication in has seen the addition of a seventh base unit, the mole , and six more derived units, the pascal for pressure, the gray , sievert and becquerel for radiation, the siemens for electrical conductance, and katal for catalytic enzymatic activity. Several units have also been redefined in terms of physical constants. Over the ensuing years, the BIPM developed and maintained cross-correlations relating various measuring devices such as thermocouples, light spectra and the like to the equivalent temperatures. The mole was originally known as a gram-atom or a gram-molecule — the amount of a substance measured in grams divided by its atomic weight.
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This was done in at the 14th CGPM. The second major trend in the post-modern SI was the migration of unit definitions in terms of physical constants of nature. Astronomers from the US Naval Observatory USNO and the National Physical Laboratory determined a relationship between the frequency of radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium atom and the estimated rate of rotation of the earth in Their atomic definition of the second was adopted in by the 13th CGPM.
By , when the second had been defined in terms of a physical phenomenon rather than the earth's rotation, the CGPM authorised the CIPM to investigate the use of the speed of light as the basis for the definition of the metre. This proposal was accepted in The candela definition proved difficult to implement so in , the definition was revised and the reference to the radiation source was replaced by defining the candela in terms of the power of a specified frequency of monochromatic yellowish-green visible light,  : which is close to the frequency where the human eye, when adapted to bright conditions, has greatest sensitivity.
After the metre was redefined in , the kilogram remained the only SI base defined by a physical artefact. During the years that followed the definitions of the base units and particularly the mise en pratique  to realise these definitions have been refined. As the IPK is the definitive kilogram, there is no way of telling whether the IPK had been losing mass or the national prototypes had been gaining mass. During the course of the century, the various national prototypes of the kilogram were recalibrated against the International Prototype Kilogram IPK and therefore against each other.
The initial starting-value offsets of the national prototypes relative to the IPK were nulled,  with any subsequent mass changes being relative to the IPK. Increasingly the use of the Boltzmann Relationship was used as the reference point and it appears likely that in the CGPM will redefine temperature in terms of the Boltzmann constant rather than the triple point of water. At its 23rd meeting , the CGPM mandated the CIPM to investigate the use of natural constants as the basis for all units of measure rather than the artefacts that were then in use.
Because the sphere will have been created so precisely, it will be able to replace the IPK. Additionally, due to its precise construction, it may be the roundest object ever created. From Wikipedia, the free encyclopedia. This article is about the development and the history of the standards used in the metric system. For history of adoption, see Metrication.
For science, see Metric system. For modern metric system, see International System of Units. The subject of this article is affected by the redefinition of the SI base units , which came into effect on 20 May As such, it may need to be revised to reflect those changes. This section is missing information about Jacques Cassini 's survey of earth completed of ; we refer to it below without defining what the survey did. Please expand the section to include this information.
Further details may exist on the talk page. January Further information: Units of measurement in France. Main article: Metre Convention. Main article: redefinition of SI base units. The names 'metre' and 'metre-system' i. Condorcet actually said, "measurement of an eternal and perfectly spherical earth is a measurement for all people for all time. His advocacy eventually resulted in him committing suicide rather than be executed by the Revolutionaries.
Iron, for example, has both high permittivity because it readily conducts electricity as well as high permeability because it makes a good magnet. A vacuum does not "conduct" electricity very well, nor can it be easily "magnetised", so the electric and magnetic constants of a vacuum are tiny. Since there is no prototype marked 8, this prototype is referred to as 8 British Library. Retrieved 10 January The Accounting Historians Journal. The Academy of Accounting Historians.
Smoot's Ear: The Measure of Humanity.
Yale University Press. Memoirs of the American Philosophical Society, Volume Bigourdan Retrieved 25 March Planetary astronomy from the Renaissance to the rise of astrophysics — Part A: tycho Brahe to Newton. Cambridge University Press. Chicago: University of Chicago Press. James Watt PDF.
Retrieved 20 October Retrieved 17 March Modern metrology; a manual of the metrical units and systems of the present century London: C Lockwood and co. Retrieved 6 February Cartography in France, — Science, Engineering, and Statecraft. University of Chicago Press. Doris January Retrieved 5 April Report upon Weights and Measures. Le systeme metrique decimal in French. Retrieved 7 February Retrieved 14 January Antoine-Laurent de Lavoisier — — Life and Works. Retrieved 4 August Retrieved 1 March Platinum Metals Rev. A Virtue of Necessity. Who needs Who needs guts or scruples when you've got power?
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