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The metric system is a decimalised system of measurement based on the metre and the gram. It exists in several variations, with different choices of base units, though these do not affect its day-to-day use. Over the last two centuries, different variants have been considered the metric system. Since the 1960s the International System of Units (SI) ("Système International d'Unités" in French, hence "SI") has been the internationally recognised standard metric system. Metric units of mass, length, and electricity are widely used around the world for both everyday and scientific purposes. __TOC__ One goal of the metric system is to have a single unit for any physical quantity. All lengths and distances, for example, are measured in metres, or thousandths of a metre (millimetres), or thousands of metres (kilometres), and so on. There is no profusion of different units with different conversion factors, such as inches, feet, yards, fathoms, rods, chains, furlongs, miles, nautical miles, leagues, etc. Multiples and submultiples are related to the fundamental unit by factors of powers of ten, so that one can convert by simply moving the decimal place: 1.234 metres is 1234 millimetres, 0.001234 kilometres, etc. The use of fractions, such as 2/5 of a metre, is not prohibited, but uncommon. Time, on the other hand, has not been metricated in everyday use: years, months, weeks, days, hours, minutes, and seconds, with non-decimal conversion factors, are used. The second and its submultiples, (e.g. microsecond), are used in scientific work, but the traditional units of time are more often used than decimal multiples of a second. The original metric system was intended to be used with the units of time of the French Republican Calendar, but these fell into disuse along with the calendar. In the late 18th century, Louis XVI of France charged a group of savants to develop a unified, natural and universal system of measurement to replace the disparate systems then in use. This group, which included such notables as Lavoisier, produced the metric system, which was then adopted by the revolutionary government of France. In the early metric system there were several fundamental or base units, the grad or grade for angles, the metre for length, the gram for mass and the litre for capacity. These were derived from each other via the properties of natural objects, mainly the Earth and water: 1 metre was originally defined as 1/40,000,000th of the polar circumference of the Earth, and 1 litre of water weighs 1 kg and measures 1 dm³. Other units were derived from this, such as the Celsius temperature scale, where water freezes at 0 °C and boils at 100 °C at standard pressure. The metre was originally defined as 1/40,000,000th of the polar circumference of the Earth, then as the length of a particular bar of platinum-iridium alloy; then in terms of the wavelength of light emitted by a specified atomic transition; and now is defined as the distance travelled by light in an absolute vacuum during 1/299,792,458 of a second. The gram, originally one millionth of the mass of a cubic metre of water, is currently defined by one thousandth of the mass of a specific object that is kept in a vault in France; however there are efforts underway to redefine it in terms of physical quantities that could be reproduced in any laboratory with suitable equipment. The second, originally one 86400th of the mean solar day and now defined by atomic clocks, is usually chosen as the base unit of time. Varying choices have been made for the third base unit, the one that is needed to incorporate the field of electromagnetics; As of 2006 this is the ampere, being the base unit of electrical current. Other quantities are derived from the base units; for example, the basic unit of speed is metres per second. As each new definition is introduced, it is designed to match the previous definition as precisely as possible, so these changes of definition have not affected most practical applications. (See SI and individual unit articles for full definitions.) The names of multiples and submultiples are formed with prefixes. They include deca- (ten), hecto- (hundred), kilo- (thousand), mega- (million), and giga- (billion); deci- (tenth), centi- (hundredth), milli- (thousandth), micro- (millionth), and nano- (billionth). The most commonly used prefixes for multiples depend on the application and sometimes tradition. For example, long distances are stated in thousands of kilometres, not megametres. Most everyday users of the metric system measure temperature in degrees Celsius, though the SI unit is the kelvin, a scale whose units have the same "size", but which starts at absolute zero. Zero degrees Celsius equals 273.15 kelvins (the word "degree" is no longer to be used with kelvins). Angular measurements have been decimalised, but the older non-decimal units of angle are far more widely used. The decimal unit, which is not part of SI, is the gon or grad, equal to one hundredth of a right angle. Subunits are named, rather than prefixed: the gon is divided into 100 decimal minutes, each of 100 decimal seconds. The traditional system, originally Babylonian, has 360 degrees in a circle, 60 minutes of arc (also called arcminutes) in a degree, and 60 seconds of arc (also called arcseconds) in a minute. The clarifier "of arc" is dropped if it is clear from the context that we are not speaking of minutes and seconds of time. Sometimes angles are given as decimal degrees, e.g., 26.4586 degrees, or in other units such as radians (especially in scientific uses other than astronomy) or angular mils. History
Goals The metric system was designed with several goals in mind. Neutral and universal The designers of the metric system meant to make it as neutral as possible so that it could be adopted universally. When the metric system was being developed, France was introducing the French Republican Calendar which was falling in disuse and was finally abolished due in part to two design faults: dates were counted from the day the French First Republic was proclaimed and the names of the months were related to purely local events, such as Brumaire (Misty), Nivose (Snowy) which did not hold true even within the French territory itself. Replicable The usual way to establish a standard was to make prototypes of the base units and distribute copies. This would make the new standard reliant on the original prototypes which would be in conflict with the previous goal since all countries would have to refer to the one holding the prototypes. The designers developed definitions of the base units such that any laboratory equipped with proper instruments should be able to make their own models of them. The original base units of the metric system could be derived from the length of a meridian of the Earth and the weight of a certain volume of pure water. They discarded the use of a pendulum since its period or, inversely, the length of the string holding the bob for the same period changes around the Earth. Likewise, they discarded using the circumference of the Earth over the Equator since not all countries have access to the Equator while all countries have access to a section of a meridian. Decimal multiples The metric system is decimal, in the sense that all multiples and submultiples of the base units are factors of powers of ten of the unit. Fractions of a unit (e.g. 29/64) are not used formally. The practical benefits of a decimal system are such that it has been used to replace other non-decimal systems outside the metric system of measurements; for example currencies. The simplicity of decimal prefixes encouraged the adoption of the metric system. Clearly the advantages of decimal prefixes derive from our using base 10 arithmetic, a consequence of our happening to have 10 digits (fingers and thumbs). At most, differences in expressing results are simply a matter of shifting the decimal point or changing an exponent; for example, the speed of light may be expressed as 299,792.458 km/s or 2.99792458×108m/s. Common prefixes All derived units would use a common set of prefixes for each multiple. Thus the prefix kilo could be used both for weight (kilogram) or length (kilometre) both indicating a thousand times the base unit. This did not prevent the popular use of names for some derived units such as the tonne which is a megagram while a quintal is accepted as 100 kilograms; both are derived from old customary units and were rounded to metric. Practical The base units were chosen to be of similar magnitude to customary units. The metre, being close to half a toise (French yard equivalent), became more popular than the failed decimal hour of the Republican Calendar which was 2.4 times the normal hour. The kilometre was originally defined as the length of an arc spanning a decimal minute of latitude, a similar definition to that of the nautical mile which was the length of an arc of one (non-decimal) minute of latitude. Coincidental similarities to real-life values Two important values, when they were expressed in the metric system, turned to be very close to a multiple of 10. The standard gravity on Earth gn has been defined to be 9.80665 m/s² exactly, which is the value at about 45° north or south of the equator. Accordingly the force exerted on a mass of one kilogram in earth gravity (F = m·a) is about ten newtons (kg-m/s²). This simplified the metrication of many machines such as locomotives, which were simply re-labeled from e.g. "85 tonnes" to "850 kN". Also, the standard atmospheric pressure, previously expressed in atmospheres, when given in pascals, is 101.325 kPa. Since the difference between 10 atmospheres and 1 MPa is only 1.3%, many devices were simply re-labelled by dividing the scale by ten, e.g. 1 atm was changed to 0.1 MPa. Another less obvious relationship is that between metres per second, and miles per hour: the ratio between these two is almost exactly equal to the square root of 5. Metric Failures A metric calendar, clock and religion were instituted by the French Revolutionary Government but were soon deemed impractical (or, in the case of the religion, ridiculous) and were quickly dropped. They are no longer considered part of the Metric system. The original French system The original French system somewhat continued the tradition of having separate base units for geometrically related dimensions, i.e. metre for lengths, are (100 m²) for areas, stere (1 m³) for dry capacities and litre (1 dm³) for liquid capacities. The hectare, equal to a hundred ares, which is the area of a square 100 metres on a side (about 2.5 acres), is still in use to measure fields. The base unit of mass is the kilogram. This is the only base unit that has a prefix, for historical reasons. Originally the kilogram was called the "grave", and the "gramme" was an alternative name for a thousandth of a grave. After the French Revolution, the word "grave" carried negative connotations, as a synonym for the title "count". The grave was renamed the kilogram *. This also serves as the prototype in the SI. It included only few prefixes from milli, one thousandth to myria ten thousand. Several national variants existed thereof with aliases for some common subdivisions. In general this entailed a redefinition of other units in use, e.g. 500-gram pounds or 10-kilometre miles. An example of these is mesures usuelles. However it is debatable whether such systems are true metric systems. Centimetre-gram-second systems Early on in the history of the metric system various centimetre gram second system of units (CGS) had been in use. These units were particularly convenient in science and technology. Metre-kilogram-second systems Later metric systems were based on the metre, kilogram and second (MKS) to improve the value of the units for practical applications. Metre-kilogram-second-coulomb (MKSC) and metre-kilogram-second-ampere (MKSA) systems are extensions of these. The International System of Units (Système international d'unités or SI) is the current international standard metric system and the system most widely used around the world. It is based on the metre, kilogram, second, ampere, kelvin, candela and mole. Metre-tonne-second systems The metre-tonne-second system of units (MTS) was based on the metre, tonne and second. It was invented in France and mostly used in the Soviet Union from 1933 to 1955. Gravitational systems Gravitational metric systems use the kilogram-force (kilopond) as a base unit of force, with mass measured in a unit known as the hyl, TME, mug or metric slug. Spelling variations Several nations, notably the United States, use the spellings meter, liter, etc. instead of metre, litre, in keeping with standard American English spelling (see also American and British English differences). In addition, the official US spelling for the SI prefix deca is deka, though it is rarely used. The spelling tonne is common outside American English, where metric ton is the normal usage. The US government has approved these spellings for official use. In scientific contexts only the symbols are used; since these are universally the same, the differences do not arise in practice in scientific use. Gram is also sometimes spelled gramme in English-speaking countries other than the United States, though that is an older spelling and its use is declining. See also | |||||||||
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