International System of Units | Wikipedia audio article

The International System of Units (SI, abbreviated
from the French Système international (d’unités)) is the modern form of the metric system, and
is the most widely used system of measurement. It comprises a coherent system of units of
measurement built on seven base units, which are the ampere, kelvin, second, metre, kilogram,
candela, mole, and a set of twenty prefixes to the unit names and unit symbols that may
be used when specifying multiples and fractions of the units. The system also specifies names
for 22 derived units, such as lumen and watt, for other common physical quantities.
The base units are derived from invariant constants of nature, such as the speed of
light in vacuum and the triple point of water, which can be observed and measured with great
accuracy, and one physical artefact. The artefact is the international prototype kilogram, certified
in 1889, and consisting of a cylinder of platinum-iridium, which nominally has the same mass as one litre
of water at the freezing point. Its stability has been a matter of significant concern,
culminating in a revision of the definition of the base units entirely in terms of constants
of nature, scheduled to be put into effect on 20 May 2019.Derived units may be defined
in terms of base units or other derived units. They are adopted to facilitate measurement
of diverse quantities. The SI is intended to be an evolving system; units and prefixes
are created and unit definitions are modified through international agreement as the technology
of measurement progresses and the precision of measurements improves. The most recent
derived unit, the katal, was defined in 1999. The reliability of the SI depends not only
on the precise measurement of standards for the base units in terms of various physical
constants of nature, but also on precise definition of those constants. The set of underlying
constants is modified as more stable constants are found, or may be more precisely measured.
For example, in 1983 the metre was redefined as the distance that light propagates in vacuum
in a given fraction of a second, thus making the value of the speed of light in terms of
the defined units exact. The motivation for the development of the
SI was the diversity of units that had sprung up within the centimetre–gram–second (CGS)
systems (specifically the inconsistency between the systems of electrostatic units and electromagnetic
units) and the lack of coordination between the various disciplines that used them. The
General Conference on Weights and Measures (French: Conférence générale des poids
et mesures – CGPM), which was established by the Metre Convention of 1875, brought together
many international organisations to establish the definitions and standards of a new system
and standardise the rules for writing and presenting measurements. The system was published
in 1960 as a result of an initiative that began in 1948. It is based on the metre–kilogram–second
system of units (MKS) rather than any variant of the CGS. Since then, the SI has been adopted
by all countries except the United States, Liberia and Myanmar.==Units and prefixes==
The International System of Units consists of a set of base units, derived units, and
a set of decimal-based multipliers that are used as prefixes. The units, excluding prefixed
units, form a coherent system of units, which is based on a system of quantities in such
a way that the equations between the numerical values expressed in coherent units have exactly
the same form, including numerical factors, as the corresponding equations between the
quantities. For example, 1 N=1 kg × 1 m/s2 says that one newton is the force required
to accelerate a mass of one kilogram at one metre per second squared, as related through
the principle of coherence to the equation relating the corresponding quantities: F=m
× a. Derived units apply to derived quantities,
which may by definition be expressed in terms of base quantities, and thus are not independent;
for example, electrical conductance is the inverse of electrical resistance, with the
consequence that the siemens is the inverse of the ohm, and similarly, the ohm and siemens
can be replaced with a ratio of an ampere and a volt, because those quantities bear
a defined relationship to each other. Other useful derived quantities can be specified
in terms of the SI base and derived units that have no named units in the SI system,
such as acceleration, which is defined in SI units as m/s2.===Base units===The SI base units are the building blocks
of the system and all the other units are derived from them. When Maxwell first introduced
the concept of a coherent system, he identified three quantities that could be used as base
units: mass, length and time. Giorgi later identified the need for an electrical base
unit, for which the unit of electric current was chosen for SI. Another three base units
(for temperature, amount of substance and luminous intensity) were added later. The early metric systems defined a unit of
weight as a base unit, while the SI defines an analogous unit of mass. In everyday use,
these are mostly interchangeable, but in scientific contexts the difference matters. Mass, strictly
the inertial mass, represents a quantity of matter. It relates the acceleration of a body
to the applied force via Newton’s law, F=m × a: force equals mass times acceleration.
A force of 1 N (newton) applied to a mass of 1 kg will accelerate it at 1 m/s2. This
is true whether the object is floating in space or in a gravity field e.g. at the Earth’s
surface. Weight is the force exerted on a body by a gravitational field, and hence its
weight depends on the strength of the gravitational field. Weight of a 1 kg mass at the Earth’s
surface is m × g; mass times the acceleration due to gravity, which is 9.81 newtons at the
Earth’s surface and is about 3.5 newtons at the surface of Mars. Since the acceleration
due to gravity is local and varies by location and altitude on the Earth, weight is unsuitable
for precision measurements of a property of a body, and this makes a unit of weight unsuitable
as a base unit.===Derived units===The derived units in the SI are formed by
powers, products or quotients of the base units and are unlimited in number. Derived
units are associated with derived quantities; for example, velocity is a quantity that is
derived from the base quantities of time and length, and thus the SI derived unit is metre
per second (symbol m/s). The dimensions of derived units can be expressed in terms of
the dimensions of the base units. Combinations of base and derived units may
be used to express other derived units. For example, the SI unit of force is the newton
(N), the SI unit of pressure is the pascal (Pa)—and the pascal can be defined as one
newton per square metre (N/m2).===Prefixes===Prefixes are added to unit names to produce
multiples and sub-multiples of the original unit. All of these are integer powers of ten,
and above a hundred or below a hundredth all are integer powers of a thousand. For example,
kilo- denotes a multiple of a thousand and milli- denotes a multiple of a thousandth,
so there are one thousand millimetres to the metre and one thousand metres to the kilometre.
The prefixes are never combined, so for example a millionth of a metre is a micrometre, not
a millimillimetre. Multiples of the kilogram are named as if the gram were the base unit,
so a millionth of a kilogram is a milligram, not a microkilogram. When prefixes are used
to form multiples and submultiples of SI base and derived units, the resulting units are
no longer coherent.The BIPM specifies twenty prefixes for the International System of Units
(SI):===Non-SI units accepted for use with SI
===Many non-SI units continue to be used in the
scientific, technical, and commercial literature. Some units are deeply embedded in history
and culture, and their use has not been entirely replaced by their SI alternatives. The CIPM
recognised and acknowledged such traditions by compiling a list of non-SI units accepted
for use with SI, which are grouped as follows: Non-SI units accepted for use with the SI
(Table 6): Certain units of time, angle, and legacy non-SI
units have a long history of consistent use. Most societies have used the solar day and
its non-decimal subdivisions as a basis of time and, unlike the foot or the pound, these
were the same regardless of where they were being measured. The radian, being 1/2π of
a revolution, has mathematical advantages but it is cumbersome for navigation, and,
as with time, the units used in navigation are largely consistent around the world. The
tonne, litre, and hectare were adopted by the CGPM in 1879 and have been retained as
units that may be used alongside SI units, having been given unique symbols. The catalogued
units are minute, hour, day, degree of arc, minute of
arc, second of arc, hectare, litre, tonne, astronomical unit
Some of the units listed in Table 7 and Table 8 are also accepted for use with the SI.
Non-SI units whose values in SI units must be obtained experimentally (Table 7):
Physicists often use units of measure that are based on natural phenomena, particularly
when the quantities associated with these phenomena are many orders of magnitude greater
than or less than the equivalent SI unit. The most common ones have been catalogued
in the SI Brochure together with consistent symbols and accepted values, but with the
caveat that their values in SI units need to be measured.
electronvolt (symbol eV), and dalton/unified atomic mass unit (Da or u)
Other non-SI units (Table 8): A number of non-SI units that had never been
formally sanctioned by the CGPM have continued to be used across the globe in many spheres
including health care and navigation. As with the units of measure in Tables 6 and 7, these
have been catalogued by the CIPM in the SI Brochure to ensure consistent usage, but with
the recommendation that authors who use them should define them wherever they are used.
bar, millimetre of mercury, ångström, nautical mile, barn, knot, neper, bel and decibel
The neper, bel and decibel have been accepted for use with the SI by the CIPM.
In the interests of standardising health-related units of measure used in the nuclear industry,
the 12th CGPM (1964) accepted the continued use of the curie (symbol Ci) as a non-SI unit
of activity for radionuclides; the SI derived units becquerel, sievert and gray were adopted
in later years. Similarly, the millimetre of mercury (symbol mmHg) was retained for
measuring blood pressure. Non-SI units associated with the CGS and the
CGS-Gaussian system of units (Table 9): The SI manual also catalogues a number of
legacy units of measure that are used in specific fields such as geodesy and geophysics or are
found in the literature, particularly in classical and relativistic electrodynamics where they
have certain advantages. The units that are catalogued are:
erg, dyne, poise, stokes, stilb, phot, gal, maxwell, gauss, and oersted.===Common notions of the metric units===
The basic units of the metric system, as originally defined, represented common quantities or
relationships in nature. They still do – the modern precisely defined quantities are refinements
of definition and methodology, but still with the same magnitudes. In cases where laboratory
precision may not be required or available, or where approximations are good enough, the
original definitions may suffice. A second is 1/60 of a minute, which is 1/60
of an hour, which is 1/24 of a day, so a second is 1/86400 of a day; a second is the time
it takes a dense object to freely fall 4.9 metres from rest.
The metre is close to the length of a pendulum that has a period of 2 seconds; most dining
tabletops are about 0.75 metre high; a very tall human (basketball forward) is about 2
metres tall. The kilogram is the mass of a litre of cold
water; a cubic centimetre or millilitre of water has a mass of one gram; a 1-euro coin,
7.5 g; a Sacagawea US 1-dollar coin, 8.1 g; a UK 50-pence coin, 8.0 g.
A candela is about the luminous intensity of a moderately bright candle, or 1 candle
power; a 60 W tungsten-filament incandescent light bulb has a luminous intensity of about
64 candela. A mole of a substance has a mass that is its
molecular mass expressed in units of grams; the mass of a mole of table salt is 58.4 g.
A temperature difference of one kelvin is the same as one degree Celsius: 1/100 of the
temperature differential between the freezing and boiling points of water at sea level;
the absolute temperature in kelvins is the temperature in degrees Celsius plus about
273; human body temperature is about 37 °C or 310 K.
A 60 W incandescent light bulb consumes 0.5 amperes at 120 V (US mains voltage) and about
0.26 amperes at 230 V (European mains voltage).==Lexicographic conventions=====
Unit names===The symbols for the SI units are intended
to be identical, regardless of the language used, but unit names are ordinary nouns and
use the character set and follow the grammatical rules of the language concerned. Names of
units follow the grammatical rules associated with common nouns: in English and in French
they start with a lowercase letter (e.g., newton, hertz, pascal), even when the symbol
for the unit begins with a capital letter. This also applies to “degrees Celsius”, since
“degree” is the unit. The official British and American spellings for certain SI units
differ – British English, as well as Australian, Canadian and New Zealand English, uses the
spelling deca-, metre, and litre whereas American English uses the spelling deka-, meter, and
liter, respectively.===Unit symbols and the values of quantities
===Although the writing of unit names is language-specific,
the writing of unit symbols and the values of quantities is consistent across all languages
and therefore the SI Brochure has specific rules in respect of writing them. The guideline
produced by the National Institute of Standards and Technology (NIST) clarifies language-specific
areas in respect of American English that were left open by the SI Brochure, but is
otherwise identical to the SI Brochure.====General rules====
General rules for writing SI units and quantities apply to text that is either handwritten or
produced using an automated process: The value of a quantity is written as a number
followed by a space (representing a multiplication sign) and a unit symbol; e.g., 2.21 kg, 7.3×102
m2, 22 K. This rule explicitly includes the percent sign (%) and the symbol for degrees
of temperature (°C). Exceptions are the symbols for plane angular degrees, minutes, and seconds
(°, ′, and ″), which are placed immediately after the number with no intervening space.
Symbols are mathematical entities, not abbreviations, and as such do not have an appended period/full
stop (.), unless the rules of grammar demand one for another reason, such as denoting the
end of a sentence. A prefix is part of the unit, and its symbol
is prepended to a unit symbol without a separator (e.g., k in km, M in MPa, G in GHz, μ in
μg). Compound prefixes are not allowed. A prefixed unit is atomic in expressions (e.g.,
km2 is equivalent to (km)2). Symbols for derived units formed by multiplication
are joined with a centre dot (⋅) or a non-breaking space; e.g., N⋅m or N m.
Symbols for derived units formed by division are joined with a solidus (/), or given as
a negative exponent. E.g., the “metre per second” can be written m/s, m s−1, m⋅s−1,
or m/s. A solidus must not be used more than once in a given expression without parentheses
to remove ambiguities; e.g., kg/(m⋅s2) and kg⋅m−1⋅s−2 are acceptable, but kg/m/s2
is ambiguous and unacceptable. The first letter of symbols for units derived
from the name of a person is written in upper case; otherwise, they are written in lower
case. E.g., the unit of pressure is named after Blaise Pascal, so its symbol is written
“Pa”, but the symbol for mole is written “mol”. Thus, “T” is the symbol for tesla, a measure
of magnetic field strength, and “t” the symbol for tonne, a measure of mass. Since 1979,
the litre may exceptionally be written using either an uppercase “L” or a lowercase “l”,
a decision prompted by the similarity of the lowercase letter “l” to the numeral “1”, especially
with certain typefaces or English-style handwriting. The American NIST recommends that within the
United States “L” be used rather than “l”. A plural of a symbol must not be used; e.g.,
25 kg, not 25 kgs. Uppercase and lowercase prefixes are not interchangeable.
E.g., the quantities 1 mW and 1 MW represent two different quantities (milliwatt and megawatt).
The symbol for the decimal marker is either a point or comma on the line. In practice,
the decimal point is used in most English-speaking countries and most of Asia, and the comma
in most of Latin America and in continental European countries.
Spaces should be used as a thousands separator (1000000) in contrast to commas or periods
(1,000,000 or 1.000.000) to reduce confusion resulting from the variation between these
forms in different countries. Any line-break inside a number, inside a compound
unit, or between number and unit should be avoided. Where this is not possible, line
breaks should coincide with thousands separators. Since the value of “billion” and “trillion”
can vary from language to language, the dimensionless terms “ppb” (parts per billion) and “ppt”
(parts per trillion) should be avoided. No alternative is suggested in the SI Brochure.====Printing SI symbols====
The rules covering printing of quantities and units are part of ISO 80000-1:2009.Further
rules are specified in respect of production of text using printing presses, word processors,
typewriters and the like.====Examples of the variety of symbols in
use around the world for kilometres per hour====The denominator “hour” (h) is often translated
to the country language: Countries with historical ties to the United
States often mix up the international “km/h” with the American “MPH”:==International System of Quantities==The quantities and equations that provide
the context in which the SI units are defined are now referred to as the International System
of Quantities (ISQ). The system is based on the quantities underlying
each of the seven base units of the SI. Other quantities, such as area, pressure, and electrical
resistance, are derived from these base quantities by clear non-contradictory equations. The
ISQ defines the quantities that are measured with the SI units. The ISQ is defined in the
international standard ISO/IEC 80000, and was finalised in 2009 with the publication
of ISO 80000-1.==Realisation of units==Metrologists carefully distinguish between
the definition of a unit and its realisation. The definition of each base unit of the SI
is drawn up so that it is unique and provides a sound theoretical basis on which the most
accurate and reproducible measurements can be made. The realisation of the definition
of a unit is the procedure by which the definition may be used to establish the value and associated
uncertainty of a quantity of the same kind as the unit. A description of the mise en
pratique of the base units is given in an electronic appendix to the SI Brochure.The
published mise en pratique is not the only way in which a base unit can be determined:
the SI Brochure states that “any method consistent with the laws of physics could be used to
realise any SI unit.” In the current (2016) exercise to overhaul the definitions of the
base units, various consultative committees of the CIPM have required that more than one
mise en pratique shall be developed for determining the value of each unit. In particular: At least three separate experiments be carried
out yielding values having a relative standard uncertainty in the determination of the kilogram
of no more than 5×10−8 and at least one of these values should be better than 2×10−8.
Both the Kibble balance and the Avogadro project should be included in the experiments and
any differences between these be reconciled. When the kelvin is being determined, the relative
uncertainty of the Boltzmann constant derived from two fundamentally different methods such
as acoustic gas thermometry and dielectric constant gas thermometry be better than one
part in 10−6 and that these values be corroborated by other measurements.==Evolution of the SI=====Changes to the SI===
The International Bureau of Weights and Measures (BIPM) has described SI as “the modern metric
system”. Changing technology has led to an evolution of the definitions and standards
that has followed two principal strands – changes to SI itself, and clarification of how to
use units of measure that are not part of SI but are still nevertheless used on a worldwide
basis. Since 1960 the CGPM has made a number of changes
to the SI to meet the needs of specific fields, notably chemistry and radiometry. These are
mostly additions to the list of named derived units, and include the mole (symbol mol) for
an amount of substance, the pascal (symbol Pa) for pressure, the siemens (symbol S) for
electrical conductance, the becquerel (symbol Bq) for “activity referred to a radionuclide”,
the gray (symbol Gy) for ionising radiation, the sievert (symbol Sv) as the unit of dose
equivalent radiation, and the katal (symbol kat) for catalytic activity.Acknowledging
the advancement of precision science at both large and small scales, the range of defined
prefixes pico- (10−12) to tera- (1012) was extended to 10−24 to 1024.The 1960 definition
of the standard metre in terms of wavelengths of a specific emission of the krypton 86 atom
was replaced with the distance that light travels in a vacuum in exactly 1/299792458
second, so that the speed of light is now an exactly specified constant of nature.
A few changes to notation conventions have also been made to alleviate lexicographic
ambiguities. An analysis under the aegis of CSIRO, published in 2009 by the Royal Society,
has pointed out the opportunities to finish the realisation of that goal, to the point
of universal zero-ambiguity machine readability.===2019 redefinitions===After the metre was redefined in 1960, the
kilogram remained the only SI base unit directly based on a specific physical artefact, the
international prototype of the kilogram (IPK), for its definition and thus the only unit
that was still subject to periodic comparisons of national standard kilograms with the IPK.
During the 2nd and 3rd Periodic Verification of National Prototypes of the Kilogram, a
significant divergence had occurred between the mass of the IPK and all of its official
copies stored around the world: the copies had all noticeably increased in mass with
respect to the IPK. During extraordinary verifications carried out in 2014 preparatory to redefinition
of metric standards, continuing divergence was not confirmed. Nonetheless, the residual
and irreducible instability of a physical IPK undermined the reliability of the entire
metric system to precision measurement from small (atomic) to large (astrophysical) scales.
A proposal was made that: In addition to the speed of light, four constants
of nature – the Planck constant, an elementary charge, the Boltzmann constant and the Avogadro
number – be defined to have exact values The International Prototype Kilogram be retired
The current definitions of the kilogram, ampere, kelvin and mole be revised
The wording of base unit definitions should change emphasis from explicit unit to explicit
constant definitions.The redefinitions were adopted at the 26th CGPM in November 2018,
and will come into effect in May 2019. The CODATA task group on fundamental constants
has announced special submission deadlines for data to compute the values that will be
announced at this event.==History=====
The improvisation of units===The units and unit magnitudes of the metric
system which became the SI were improvised piecemeal from everyday physical quantities
starting in the mid-18th century. Only later were they moulded into an orthogonal coherent
decimal system of measurement. The degree centigrade as a unit of temperature
resulted from the scale devised by Swedish astronomer Anders Celsius in 1742. His scale
counter-intuitively designated 100 as the freezing point of water and 0 as the boiling
point. Independently, in 1743, the French physicist Jean-Pierre Christin described a
scale with 0 as the freezing point of water and 100 the boiling point. The scale became
known as the centi-grade, or 100 gradations of temperature, scale.
The metric system was developed from 1791 onwards by a committee of the French Academy
of Sciences, commissioned to create a unified and rational system of measures. The group,
which included preeminent French men of science, used the same principles for relating length,
volume, and mass that had been proposed by the English clergyman John Wilkins in 1668
and the concept of using the Earth’s meridian as the basis of the definition of length,
originally proposed in 1670 by the French abbot Mouton. In March 1791, the Assembly adopted the committee’s
proposed principles for the new decimal system of measure including the metre defined to
be 1/10,000,000th of the length of the quadrant of earth’s meridian passing through Paris,
and authorised a survey to precisely establish the length of the meridian. In July 1792,
the committee proposed the names metre, are, litre and grave for the units of length, area,
capacity, and mass, respectively. The committee also proposed that multiples and submultiples
of these units were to be denoted by decimal-based prefixes such as centi for a hundredth and
kilo for a thousand. Later, during the process of adoption of the
metric system, the Latin gramme and kilogramme, replaced the former provincial terms gravet
(1/1000 grave) and grave. In June 1799, based on the results of the meridian survey, the
standard mètre des Archives and kilogramme des Archives were deposited in the French
National Archives. Subsequently, that year, the metric system was adopted by law in France.
The French system was short-lived due to its unpopularity. Napoleon ridiculed it, and in
1812, introduced a replacement system, the mesures usuelles or “customary measures” which
restored many of the old units, but redefined in terms of the metric system.
During the first half of the 19th century there was little consistency in the choice
of preferred multiples of the base units: typically the myriametre (10000 metres) was
in widespread use in both France and parts of Germany, while the kilogram (1000 grams)
rather than the myriagram was used for mass.In 1832, the German mathematician Carl Friedrich
Gauss, assisted by Wilhelm Weber, implicitly defined the second as a base unit when he
quoted the Earth’s magnetic field in terms of millimetres, grams, and seconds. Prior
to this, the strength of the Earth’s magnetic field had only been described in relative
terms. The technique used by Gauss was to equate the torque induced on a suspended magnet
of known mass by the Earth’s magnetic field with the torque induced on an equivalent system
under gravity. The resultant calculations enabled him to assign dimensions based on
mass, length and time to the magnetic field.A candlepower as a unit of illuminance was originally
defined by an 1860 English law as the light produced by a pure spermaceti candle weighing
​1⁄6 pound (76 grams) and burning at a specified rate. Spermaceti, a waxy substance
found in the heads of sperm whales, was once used to make high-quality candles. At this
time the French standard of light was based upon the illumination from a Carcel oil lamp.
The unit was defined as that illumination emanating from a lamp burning pure rapeseed
oil at a defined rate. It was accepted that ten standard candles were about equal to one
Carcel lamp.===Metre Convention===A French-inspired initiative for international
cooperation in metrology led to the signing in 1875 of the Metre Convention, also called
Treaty of the Metre, by 17 nations. Initially the convention only covered standards for
the metre and the kilogram. In 1921, the Metre Convention was extended to include all physical
units, including the ampere and others thereby enabling the CGPM to address inconsistencies
in the way that the metric system had been used.A set of 30 prototypes of the metre and
40 prototypes of the kilogram, in each case made of a 90% platinum-10% iridium alloy,
were manufactured by British metallurgy specialty firm and accepted by the CGPM in 1889. One
of each was selected at random to become the International prototype metre and International
prototype kilogram that replaced the mètre des Archives and kilogramme des Archives respectively.
Each member state was entitled to one of each of the remaining prototypes to serve as the
national prototype for that country.The treaty also established a number of international
organisations to oversee the keeping of international standards of measurement:===The cgs and MKS systems===In the 1860s, James Clerk Maxwell, William
Thomson (later Lord Kelvin) and others working under the auspices of the British Association
for the Advancement of Science, built on Gauss’ work and formalised the concept of a coherent
system of units with base units and derived units christened the centimetre–gram–second
system of units in 1874. The principle of coherence was successfully used to define
a number of units of measure based on the CGS, including the erg for energy, the dyne
for force, the barye for pressure, the poise for dynamic viscosity and the stokes for kinematic
viscosity.In 1879, the CIPM published recommendations for writing the symbols for length, area,
volume and mass, but it was outside its domain to publish recommendations for other quantities.
Beginning in about 1900, physicists who had been using the symbol “μ” (mu) for “micrometre”
or “micron”, “λ” (lambda) for “microlitre”, and “γ” (gamma) for “microgram” started to
use the symbols “μm”, “μL” and “μg”.At the close of the 19th century three different
systems of units of measure existed for electrical measurements: a CGS-based system for electrostatic
units, also known as the Gaussian or ESU system, a CGS-based system for electromechanical units
(EMU) and an International system based on units defined by the Metre Convention. for
electrical distribution systems. Attempts to resolve the electrical units in
terms of length, mass, and time using dimensional analysis was beset with difficulties—the
dimensions depended on whether one used the ESU or EMU systems. This anomaly was resolved
in 1901 when Giovanni Giorgi published a paper in which he advocated using a fourth base
unit alongside the existing three base units. The fourth unit could be chosen to be electric
current, voltage, or electrical resistance. Electric current with named unit ‘ampere’
was chosen as the base unit, and the other electrical quantities derived from it according
to the laws of physics. This became the foundation of the MKS system of units.
In the late 19th and early 20th centuries, a number of non-coherent units of measure
based on the gram/kilogram, centimetre/metre and second, such as the Pferdestärke (metric
horsepower) for power, the darcy for permeability and “millimetres of mercury” for barometric
and blood pressure were developed or propagated, some of which incorporated standard gravity
in their definitions.At the end of the Second World War, a number of different systems of
measurement were in use throughout the world. Some of these systems were metric system variations;
others were based on customary systems of measure, like the U.S customary system and
Imperial system of the UK and British Empire.===The Practical system of units===
In 1948, the 9th CGPM commissioned a study to assess the measurement needs of the scientific,
technical, and educational communities and “to make recommendations for a single practical
system of units of measurement, suitable for adoption by all countries adhering to the
Metre Convention”. This working document was Practical system of units of measurement.
Based on this study, the 10th CGPM in 1954 defined an international system derived from
six base units including units of temperature and optical radiation in addition to those
for the MKS system mass, length, and time units and Giorgi’s current unit. Six base
units were recommended: the metre, kilogram, second, ampere, degree Kelvin, and candela.
The 9th CGPM also approved the first formal recommendation for the writing of symbols
in the metric system when the basis of the rules as they are now known was laid down.
These rules were subsequently extended and now cover unit symbols and names, prefix symbols
and names, how quantity symbols should be written and used and how the values of quantities
should be expressed.===Birth of the SI===In 1960, the 11th CGPM synthesised the results
of the 12-year study into a set of 16 resolutions. The system was named the International System
of Units, abbreviated SI from the French name, Le Système International d’Unités.==See also====Notes

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