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The metric system was developed during the French Revolution to replace the various measures previously used in France. The metre (spelled "meter" in American English) is the unit of length in the metric system and was originally based on the dimensions of the earth, as far as it could be measured at the time. The litre (or in American English "liter"), is the unit of volume and was defined as one thousandth of a cubic metre. The metric unit of mass is the kilogram and it was defined as the mass of one litre of water. The metric system was, in the words of French philosopher Marquis de Condorcet, "for all people for all time". The metric system has names to cover different ranges of the same measure. Instead of using names based on the context of the measure, the metric system mainly uses names made by adding prefixes, such as kilo or milli, as decimal multipliers to the base unit names. Thus, one kilogram is 1000 grams and one kilometre is 1000 metres. During the nineteenth century the metric system was adopted by both the worldwide scientific community and many countries as the system of measurement. It therefore became truly international. Until 1875 the French government owned the prototype metre and kilogram, but in that year the Convention of the metre was signed and control of the standards relating to mass and length passed on to a trio of intergovernment organisations. In 1960 the metric system was extensively revised to form the International System of Units, abbreviated to "SI".
On the eve of the French Revolution , France had an estimated quarter of a million different units of measurement. In many cases the value of a unit differed from town to town and even from trade to trade even though they might have the same name. While certain standards, such as the pied du roi (the King's foot) had a degree of preeminence and were used by savants (scientists), many traders used their own measuring devices. This gave scope for fraud and hindered commerce and industry.^{[1]} The metric system was designed to replace this confusion with a radical new system with fixed values.^{[2]}
In France^{[1]} and the rest of Europe there was a multitude of measurement units. The differences were like those between United States customary units and United Kingdom imperial units of liquid volume measures – a US pint consists of 16 US fluid ounces while an imperial pint is 20 UK fluid ounces and the US fluid ounce is about 4% larger than the UK fluid ounce. Differences such as these occurred across Europe.
Between 1790 and 1800, during the French Revolution , and with the backing of Louis XVI, the system of weights and measures was totally reformed.^{[3]} The new system of measures had a rational mathematical basis and was part of the radical effort to sweep away old traditions and conventions and replace them with something new and better.^{[4]} The French philosopher, the Marquis de Condorcet, who was one of those entrusted by Louis XVI to overhaul the system of measurement, characterised the metric system as "for all people for all time".^{[5]}
The key units of the republican measures system were:
Since it was not practical to determine the metre and the kilogram with adequate precision and repeatability, it was decided to use artefacts as the reference kilogram and metre, against which instruments could be calibrated. The mètre des Archives and kilogramme des Archives were manufactured to meet these definitions as closely as possible. The definition of the metre has since been revised to be independent of any artifact,^{[8]} and a similar redefinition of the kilogram occurred in May 2019.
The new system was not popular and people continued to use their customary measures. Napoleon recognised the value of a sound basis for a system of measurement but ridiculed the metric system. In 1812 he introduced the mesures usuelles, a modification of the metric measures for use in small retail businesses. These mesures usuelles used some older unit names but used the metre des Archives and the kilogramme des Archives as its basis for measurement. However, all government, legal and similar works still had to use the metric system and the metric system continued to be taught at all levels of education.^{[9]} This system survived in France until the metric system was reinstated for all purposes in 1840.
The metric system developed as the understanding of science and in measuring techniques have advanced. In 1875, the Convention of the metre was signed and control of the metric system passed from France to a trio of intergovernment organisations headed by the Conférence générale des poids et mesures (CGPM) and based in Sèvres, France.^{[10]} In 1960, at the 11th conference of the CGPM, the metric system was overhauled and the resultant system named "The International System of Units", (also known as "SI", an abbreviation Système international d'unités).^{[11]}
The driving force behind the metric system was the need for a single, rational and universal system of weights and measures that could be used worldwide.
The names of the units of measure used in the metric system consist of two parts: a unit name (for example "metre", "gram", "litre") and an associated multiplier prefix (for example "milli" meaning ^{1}⁄_{1000}, "kilo" meaning 1000). The result is that there are a variety of different named units available to measure the same quantity (for example 10 millimetres = 1 centimetre, 100 centimetres = 1 metre, 1000 metres = 1 kilometre). Each unit and each prefix has a standard symbol (not abbreviation) associated with it.
In 1861, during discussions about standardising electrical units of measure, Charles Bright and Latimer Clark proposed that these units be named, not in relation to what they are used for, or common objects, but after eminent scientists; with the electrical units of resistance, potential difference and capacitance being named the ohm, volt and farad in honour of Georg Ohm, Alessandro Volta and Michael Faraday respectively. This proposal had the support of William Thomson (Lord Kelvin)^{[12]} who had been instrumental in forming the Committee of Electrical Standards of the British Association for the Advancement of Science. The use of scientists' names for such units was subsequently extended to other units, including the watt named after James Watt and the degree Celsius named after Anders Celsius.
Historically, individual units evolved which were based on the size and context of what was being measured. These units could avoid the need to use large numbers of smaller units or small numbers of larger units for a measurement. These units were generally defined as a convenient multiple of a smaller unit or a convenient division of a larger unit. Thus, in prerevolutionary France, the inch was divided into 12 lines and each line was subdivided into 12 points.^{[13]} Similarly, astronomers introduced the lightyear to describe large distances. The metric system on the other hand uses prefixes to denote multipliers of the one basic unit. For example, the prefix "kilo" is used to denote a multiplier of 1000; thus one kilometre is 1000 metres, one kilogram is 1000 grams, and one kilowatt is 1000 watts.
Each unit and each prefix in the metric systems has been allocated a unique symbol by the CGPM. Unlike abbreviations which are a contraction of the local word for the unit in question, and which can therefore differ from one language to another, SI symbols are a form of standardised mathematical notation to represent the units and are the same in any language (compare chemical symbols).
There are certain circumstances where abbreviations (as opposed to the SI symbols) are used, particularly where safety is concerned. One such instance is the use of "mcg" rather than "μg" to represent "micrograms" in the pharmaceutical industry.^{[14]}
There are three basic classes of units in SI:
This article will not differentiate between these various classes of units, other than to make references to them as appropriate.
The metre is the base unit of length. Its name was derived from the Greek μέτρον καθολικόν (métron katholikón), "a universal measure". This word gave rise to the French mètre which was subsequently introduced into the English language.^{[19]}
The metre is defined as the length of the path travelled by light in a vacuum in 1/299792458 of a second
Originally the metre was to have been one ten millionth of the distance between the North Pole and the equator. The French Academy of Sciences commissioned an expedition led by Jean Baptiste Joseph Delambre and Pierre Méchain, lasting from 1792 to 1799, which measured the distance between the Dunkerque belfry and Montjuïc castle, Barcelona to estimate the length of the meridian arc through Dunkerque (assumed to be the same length as the Paris meridian). This portion of the meridian was to serve as the basis for the length of the half meridian, connecting the North Pole with the equator.^{[20]} In 1799 a metre bar was manufactured based on results of this survey. Although the bar was subsequently found to be 0.02% shorter than it should have been, the metre has always been based on the length of the bar rather than the half meridian.
Metric Unit 
Imperial/ Customary Equivalent 
Visualisation 

1 km  0.62 miles 1100 yd 
The Mall (links Trafalgar Square and Buckingham Palace) Niagara Falls (Bank to bank) 
100 m  110 yd  Length of a gridiron football (Canadian), association football (soccer) or rugby field Length of fourcoach train 
10 m  33 ft  Width of a tennis court (10.97 m) 
1 m 100 cm 
1.1 yd 3.3 ft 40 in 
Length of a baseball bat (maximum = 1.067 m) Length of a cricket bat (maximum = 0.965 m) 
10 cm  4 in  Width of a man's palm 
10 mm 1 cm 
^{2}⁄_{5} in  Width of an average acorn 
1 mm  0.04 in  Thickness of denim cloth^{[21]} 
100 μm  0.004 in  Thickness of a sheet of photocopier paper 
10 μm  0.0004 in  Thickness of plastic cling wrap 
The SI unit of area is the square metre (m^{2}), but when the metric system was first introduced in 1795, the unit of land measure was defined as the are, being 100 m^{2} (or the area equivalent to that of a square having sides of 10 m). This measure was only used in a few countries, but the hectare (100 ares or 10,000 m^{2}), is a nonSI unit that has been catalogued as being acceptable for use with the SI and is in widespread use throughout the world. (A hectare is about 2.5 acres.)
The SI unit of volume is the cubic metre (m^{3}) – the volume equivalent to the space occupied by a cube with sides of one metre. However, the litre, one of the oldest metric units, having been formally defined in 1795 as the volume occupied by a cube with sides of one tenth of a metre^{[4]} (making it equal to 0.001 m^{3}, or 1 dm^{3}) is in widespread use. The litre is not technically part of SI, but its use is sufficiently widespread that it is "accepted for use within SI".^{[17]}
SI distinguishes between mass and weight – mass being a measure of the amount of material contained in an object and weight the gravitational force on that object. We normally "weigh" objects by comparing the gravitational force on that object with the gravitational force on an object of known mass (such as a 1 kg "weight"). Although this concept was understood by ancient scientists (Archimedes' principle is based on it), the wording was only formalised in 1901.
The SI base unit of mass is the kilogram which is defined in terms of three fundamental physical constants: The speed of light c, a specific atomic transition frequency Δν_{Cs}, and the Planck constant h. The formal definition is:
The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015×10^{−34} when expressed in the unit J⋅s, which is equal to kg⋅m^{2}⋅s^{−1}, where the metre and the second are defined in terms of c and Δν_{Cs}.
Originally it was defined in 1795 as the mass of one litre of water at the temperature of melting ice (0 °C),^{[22]} though to ensure greater consistency of kilogram artefacts, and to create a practical physical realisation of the kilogram, a platinum artefact intended to have the mass of precisely 1 kg was manufactured and placed in the French Archives in 1799. This artefact was replaced by one of British manufacture in 1889 which was the definitive kilogram until May 2019.
The kilogram is dissimilar to the other SI base units in that it is expressed as a multiple of another unit (the gram) with a multiplier prefix ("kilo") added to it. A teaspoon holds about 5 grams of sugar which makes milligrams or in some case micrograms convenient units to measure medicine doses when they are dispensed in capsules. The ton, variously defined, had long been a customary unit of measure for large masses and in the midnineteenth century the metric ton (or tonne) of 1000 kg (i.e. equivalent to the megagram) was introduced. Although the tonne is not an SI Unit, its continued use in many countries has led to it being "accepted for use within SI".^{[17]}
Metric Unit 
Imperial/ Customary Equivalent 
Visualisation 

100 Gg (100,000 tonnes) 
98,000 long tons (UK) 110,000 short tons (US) 
Washington Monument in Washington, DC 82,400 tonnes (81,100 long tons; 90,800 short tons), half above ground 
10 Gg (10,000 tonnes) 
9,800 long tons 11,000 short tons 
Power shovel Marion 6360 weighs 11,500 tonnes (11,300 long tons; 12,700 short tons) 
1,000 Mg, 1 Gg (1,000 tonnes) 
980 long tons 1100 short tons 
Large mining excavator Caterpillar 6090 FS weighs 980 tonnes (960 long tons; 1,080 short tons) 
100,000 kg, 100 Mg (100 tonnes) 
98 long tons 110 short tons 
Australian triple road train Gross weight of a triple is 115 tonnes (113 long tons; 127 short tons) 
10,000 kg, 10 Mg (10 tonnes) 
22,000 lb 9.8 long tons 11 short tons 
Fully laden box truck; small lorry 
1,000 kg, 1 Mg (1 tonne, 1 metric ton) 
2200 lb 0.98 long tons 1.1 short tons 
Small motor car – typically powered by an engine of between 1.0 and 1.2 L 
100 kg  15 stone 11 lb (UK) 220 lb (US) 
Large man – about 15% of US Caucasian males exceed 100 kg^{[23]} 
10 kg  22 lb  Average weight of a 12monthold child^{[24]} 
1 kg  2.2 lb; 2 lb 3 oz  One litre drink (excluding the weight of the container) 
100 g  3^{1}⁄_{2} oz  Midway between a tennis ball (≈58 g) and a cricket ball (≈160 g) or a baseball (≈145 g) 
10 g  ^{3}⁄_{8} oz  A large coin

1 g  15 grains  Three standard paperclips (1.1 g); a plastic pen cap; two peanut seeds^{[26]} 
100 mg  1.5 grains  Low dose (81 mg) enteric coated aspirin tablet – 120 mg with binders 
10 mg  0.15 grains  One third of a standard paper staple 
The metre was originally defined to be one ten millionth of the distance between the North Pole and the Equator through Paris. https://handwiki.org/wiki/index.php?curid=2005507
Definition of a hectare and of an are. https://handwiki.org/wiki/index.php?curid=2088652
One litre is equivalent to this cube, Each side is 10 cm, 1 litre water ≈ 1 kilogram water. https://handwiki.org/wiki/index.php?curid=2045041
The degree Celsius (symbol: °C) came into use in its present form in 1744 when 0 °C was defined as the freezing point of water and 100 °C was defined as the boiling point of water, both at a pressure of one standard atmosphere.
Before 1948 the unit was known as "centigrade" from the Latin "centum" translated as 100 and "gradus" translated as "steps". However, in France and Spain, the word "centigrade" also meant 0.0001 of a right angle. To avoid confusion, the BIPM and other standards first referred to the degree centigrade as the "centesimal degree" but in 1948, the CGPM changed the name to "degree Celsius", in honour of the Swedish scientist Anders Celsius who first proposed a similar scale (though Celsius' scale had 0 and 100 switched around). However, they retained the symbol °C.
Temperature point  Metric  Imperial/ Customary 

Sublimation point of dry ice (frozen CO_{2}) at standard atmospheric pressure  −78 °C  –108 °F 
Melting point of ice  0 °C  32 °F 
Normal human body temperature  37.0 °C  98.6 °F 
Boiling point of water at standard atmospheric pressure  100 °C  212 °F 
When the metric system was first introduced in 1795, all metric units could be defined by reference to the standard metre or to the standard kilogram. In 1832 Carl Friedrich Gauss, when making the first absolute measurements of the Earth's magnetic field, needed standard units of time alongside the units of length and mass. He chose the second (rather than the minute or the hour) as his unit of time, thereby implicitly making the second a base unit of the metric system.^{[27]} The hour and minute have however been "accepted for use within SI".^{[17]} One second is now defined by taking the fixed numerical value of the caesium frequency ∆ν_{Cs}, the unperturbed groundstate hyperfine transition frequency of the caesium133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s^{−1}.
During the 20th century it became apparent that the Earth's rotation was slowing down. This results in days becoming 1.4 milliseconds longer each century.^{[28]} It was verified by comparing the calculated locations of eclipses of the Sun with those observed in antiquity going back to Chinese records of 763 BC^{[29]} and Roman records of AD 484. The "sunrise" point of the eclipse on 14 January 484 was backcalculated and, using 20th century data, should have been close to Lisbon. Ancient records however record the "sunrise point" as being in the Ionian Sea, off the coast of Greece. This difference can be accounted for by assuming that the Earth is slowing down, and as a result a day in Roman times was a little over 0.02 seconds shorter than today.^{[30]}
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 auspices of the International Astronomical Union (IAU) took the lead in maintaining the standards relating to time.^{[31]} In 1958, in anticipation of technology being able to measure the rate at which the Earth is slowing down, it was agreed that the second would be defined on the basis that in 1900 the Earth's average rotational speed gave an average day of exactly 60×60×24 = 86,400 seconds.^{[28]} Astronomers from the US Naval Observatory (USNO) and the National Physical Laboratory determined a relationship between a specific microwave frequency emitted by an excited caesium133 atom (about 9 GHz) and the backcalculated rate of rotation of the Earth in 1900. Their value was adopted in 1968 by the 13th CGPM as being the definition of the second.
During the 19th century, the British Association for the Advancement of Science took the lead in standardising units of measurement used in science and technology across the globe. Under the leadership of men like James Clerk Maxwell and Lord Kelvin, the metric system was the system of choice. Some units that they developed are still in use today; others have been superseded.
Scientists and engineers subsequently developed many other units of measure, some of which were discarded with the coming of SI. Scientific and technical units of measure frequently encountered by the layman today include:
The metric system of measure was first given a legal basis in 1795 by the French Revolution ary government. Article 5 of the law of 18 Germinal, Year III (7 April 1795) defined five units of measure.^{[4]}
By 1870 the metric system had been adopted by most of the countries of Europe and on 20 May 1875 an international treaty known as the Convention du Mètre (Metre Convention) was signed by 17 to harmonise measurements between the states.^{[32]}^{[33]} Initially the treaty only provided for the coordination of length and mass, but in 1921 the treaty was extended to cover all types of measurement. The treaty established the following organisations to conduct international activities relating to a uniform system for measurements:
In 1889 sets of new international prototype metres and kilograms made from a 90% platinum, 10% iridium alloy were manufactured by the London firm Johnson Matthey and delivered to the CGPM who calibrated them against the 1799 prototype. One master copy and a set of working copies were retained by the BIPM and the rest distributed to member nations. At intervals of about 25 years each nation returned their copies for recalibration against the master copies.^{[34]}
In 1921 the mandate for the CGPM and its subsidiary organisations was extended to include the standardisation of all physical measurements including electrical measurements, time and temperature.