A Brief History of SI Units

Nations which traded and exchanged scientific ideas with each other had started realising the importance and necessity of standardisation of weights and measures which became much more apparent by the mid-eighteenth century. In 1790 France sent a proposal to Britain and the United States to establish a uniform measure of length based on the period of a pendulum. Since the time period of a pendulum depends on the acceleration of gravity which varies from place to place, it was necessary to fix a latitude for the definition. But the French proposal was rejected by both Britain and United States because they could not agree on the latitude to be chosen for the definition. Needless to say, they could not reach an agreement because every one wanted a definition according to a latitude passing through their own country.

After the proposal of France was rejected by Britain and United States, France unilaterally constituted a panel of five leading French scientists to investigate weights and measures. And the proposals of the panel were accepted by the French Assembly on 30 March 1791.

One of the decisions of the panel was that the measure of length should be equal to one ten-millionth of the length of a meridian arc passing through Paris from the North Pole to the Equator. Panel also proposed gramme as the unit of mass which was equal to the mass of one cubic centimetre of water.

The unit gramme, however, was too small for most of the practical purposes, so it was prefixed with ‘kilo’ to make it ‘kilogramme’ in 1792. And, today ‘kilogram’ happen to be only base unit in SI Units that has a prefix as part of its unit name.

The task of measuring the meridian arc to define length was completed by two astronomers Pierre Méchain and Jean-Baptiste Delambre in around six years from 1792 to 1798. Consequently, the final value of the metre was computed in 1799 using the data submitted by these astronomers.

After the final value of metre was determined, platinum prototypes of length, known as metre des archives and mass, known as kilogramme des archives were fabricated and in 1799 the metric system (which constitutes units, and decimal multiples and fractions of the units) based on these units became by law the sole system of weights and measures in France.

In 1832 German mathematician Carl-Friedrich Gauss defined and used second as the unit of time. He defined second by partitioning one solar day into two cycles of 12 periods, each period divided into 60 intervals, and each interval divided again into 60 more intervals. Thus, second was 1/86400th of the day.

As time passed, the metre des Archives as the unit of distance continued to be adopted by many nations. Meanwhile, various units for force, work, and energy; units for electrical quantities, and units for thermodynamic quantities were also developed.

The definition of meter based on the length of the meridian passing through Paris, which during its inception was assumed to ensure international reproducibility, was, however, so impractical that it was widely felt that it should be revised. Consequently, an international treaty known as the Meter Convention (Convention du Metre) was signed by 17 states on 20 May 1875 in Paris. (To commemorate this event, we now celebrate 20th May of every year as the World Metrology Day.) This treaty established following three organisations to conduct activities related to measurements:

  1. General Conference on Weights and Measures (in short known as CGPM for its French name Conférence générale des poids et mesures): It is an intergovernmental conference of official delegates of member nations which acts on behalf of the governments of its members and acts as the supreme authority for all actions related to metrology.
  2. International Committee for Weights and Measures (in short known as CIPM for its French name Comité international des poids et mesures): It consists of selected scientists and metrologists and executes the decisions of the CGPM. It also supervises the International Bureau of Weights and Measures.
  3. International Bureau of Weights and Measures (in short known as BIPM for its French name Bureau international des poids et mesures): It is a permanent laboratory and world centre of scientific metrology. Its activities include the establishment of the basic standards and scales of the principal physical quantities and maintenance of the international prototype standards.

The task of the BIPM is to ensure worldwide unification of measurements by ensuring all of the followings:

  • To establish fundamental standards and scales for the measurement of the principal physical quantities and maintain the international prototypes.
  • To carry out comparisons of national and international standards.
  • To ensure the coordination of corresponding measurement techniques.
  • To carry out and coordinate measurements of the fundamental physical constants relevant to these activities.

The BIPM operates under the exclusive supervision of the International Committee for Weights and Measures (CIPM) which itself comes under the authority of the General Conference on Weights and Measures (CGPM) and reports to it on the work accomplished by the BIPM.

Delegates from all Member States attend the General Conference organized by CGPM which, at present, is organized every four years. The function of General Conference is:

  • To discuss and initiate the arrangements required to ensure the propagation and improvement of the International System of Units (SI).
  • To confirm the results of new fundamental metrological determinations and various scientific resolutions of international scope.
  • To take all major decisions concerning the finance, organization and development of the BIPM.

Under the terms of the Metre Convention, new international prototypes of metre and kilogram were manufactured and formally adopted by the first General Conference on Weights and Measures (CGPM) in 1889. This system of units was known as the Metric System of units. In 1960, the 11th CGPM formally renamed it as the International System of Units, known as the SI Units for its French name Système International d’Unités. In following paragraphs, noteworthy resolutions/outcomes of all the CGMP meetings have been enlisted. Resolutions/Outcomes mentioned for each CGPM are not exhaustive; only those are mentioned which can be useful to beginners.

1st CGPM (1889)

  • A cylinder made of platinum-iridium, known as the International Prototype Kilogram (IPK), decided as the standard kilogram.
  • An X-cross-section bar also made from platinum-iridium, known as the International Prototype Metre (IPM), decided as the standard metre.
  • Working copies of both the artifacts selected.
  • Copies of IPK and IPM distributed to member nations.

3rd CGPM (1901)

  • The litre was defined as the volume occupied by a mass of 1 kilogram of pure water, at its maximum density and at standard atmospheric pressure.
  • Ambiguity between mass and weight were removed. It was clarified that (a) the kilogram is the unit of mass which is equal to the mass of the international prototype of the kilogram (IPK), and (b) the weight denotes a quantity of the same nature as a force: the weight of a body is the product of its mass and the acceleration due to gravity.
  • The value for the standard acceleration due to gravity was decided.

7th CGPM (1927)

  • The metre redefined. According to new definition: The unit of length is the meter, defined by the distance, at 0°, between the axes of the two central lines marked on the bar of platinum-iridium kept at the BIPM, and declared Prototype of the meter by the 1st CGPM, this bar being subject to standard atmospheric pressure and supported on two cylinders of at least one centimeter diameter, symmetrically placed in the same horizontal plane at a distance of 571 mm from each other.
  • Writing of unit symbols: It was decided that Roman (upright) type, in general lower case, is used for symbols of units; if, however, the symbols are derived from proper names, capital roman type is used. These symbols are not followed by a full stop.
  • Writing of unit numbers: It was decided that in numbers, the comma (French practice) or the dot (British practice) is used only to separate the integral part of numbers from the decimal part. Numbers may be divided in groups of three in order to facilitate reading; neither dots nor commas are ever inserted in the spaces between groups.

8th CGPM (1933)

  • Establishment of a Photometric Advisory Committee.
  • Substitution of absolute electrical units to so-called “international” units.

9th CGPM (1948)

  • The ampere, unit of current, defined.
  • It was agreed that the thermometric scale based on the triple point of water would be more accurate than the melting point of the ice. Consequently, the zero of the thermodynamic scale was defined as the temperature 0.0100 degrees lower than that of the triple point of pure water. The thermodynamic scale itself was to be defined later.
  • An absolute thermodynamic scale with only one fundamental fixed point, the triple point of pure water was accepted.
  • The unit of heat quantity was defined as joule.

10th CGPM (1954)

  • CGPM decided to define the thermodynamic scale of temperature by means of the triple point of water as the fundamental fixed point, assigning it the temperature exactly \(273.16\) degrees Kelvin.

11th CGPM (1960)

  • The metre was redefined as the length equal to \(1650763.73\) wavelengths in the vacuum of the radiation corresponding to the transition between the levels \(2p_{10}\) and \(5d_5\) of the krypton atom 86.
  • The second was defined as the \(1/31\) fraction \(556 925.9747\) of the tropical year for 1900 January 0 to 12 hours of ephemeris time.
  • The metric system of units given the official name Système International d’Unités abbreviated as SI Units.

12th CGPM (1964)

  • Repeals the definition of the liter given in 1901 by the Third General Conference of Weights and Measures; declares that the word “liter” can be used as a special name given to the cubic decimeter; and recommends that the liter name not be used to express the results of high precision volume measurements.

13th CGPM (1967)

  • Redefined the second as the duration of \(9192631770\) periods of radiation corresponding to the transition between the two hyperfine levels of the ground state of the atom of cesium 133.
  • The Degree Kelvin renamed as kelvin.
  • The candela defined as the luminous intensity, in the perpendicular direction, of a surface of (1/600000) square meter of a black body at the freezing temperature of platinum under the pressure of (101325) newtons per square meter.

14th CGPM (1971)

  • The mole defined as the quantity of matter in a system containing as many elementary entities as there are atoms in 0.012 kilogram of carbon-12

15th CGPM (1975)

The becquerel, defined as equal to the second to the power minus one, and the gray, defined as equal to joule per kilogram adopted as the units of ionizing radiation.

16th CGPM (1979)

  • Redefined candela as the luminous intensity, in a given direction, from a source that emits monochromatic radiation of frequency \(540 \times 10^{12}\) hertz and has a radiant intensity in that direction is \(1/683\) watt per steradian.

17th CGPM (1983)

  • Redefined the meter as the length of the path travelled in the vacuum by the light during a period of \(1/299792458\) of a second by fixing the speed of light in vacuum as \(299792458\) m/s.

20th CGPM (1995)

  • A resolution related to monitoring the stability of the international prototype of the kilogram passed.
  • Interpreted supplementary units in the SI, namely the radian and the steradian, as dimensionless derived units and, consequently, eliminated the class of supplementary units as a separate class in the SI.

21st CGPM (1999)

  • Recommended that national laboratories continue their efforts to refine experiments that link the unit of mass to fundamental or atomic constants with a view to a future redefinition of the kilogram.

Note: This recommendation is being mentioned here because it paid off as a new definition of kilogram in terms of Plank’s constant, a fundamental constant, in 26th CGPM held in 2018.

22nd CGPM (2003)

  • Reaffirms a resolution of the 9th CGPM, 1948 in which it decided that the symbol for the decimal marker shall be either a point or a comma and the numbers may be divided in groups of three in order to facilitate reading; neither dots nor commas are ever inserted in the spaces between groups.

26th CGPM (2018)

  • Of the seven base units of the SI, only the kilogram was still defined in terms of a material artifact – the international prototype of the kilogram – and the definitions of the ampere, mole, and candela depended on the kilogram. In one of the most important revisions of SI Units, 26th CGPM decided that effective from 20 May 2019, the International System of Units is the system of units in which:
  1. The unperturbed ground state hyperfine transition frequency of the caesium 133 atom, \(\Delta \nu_{Cs}\) is \(9 192 631 770\) \(Hz\).
  2. The speed of light in vacuum, (c) is (299 792 458) (m/s).
  3. The Planck constant, (h) is (6.626 070 15 \times 10^{–34}) (J s).
  4. The elementary charge (e) is (1.602 176 634 \times 10^{–19}) (C).
  5. The Boltzmann constant (k) is (1.380 649 \times 10^{–23}) (J/K).
  6. The Avogadro constant (N_A) is (6.022 140 76 \times 10^{23}) (mol^{–1}).
  7. The luminous efficacy of monochromatic radiation of frequency (540 \times 10^{12}) (Hz), (K_{cd}), is (683) (lm/W).

For more details, please read New SI Units Effective Since 20th May 2019.

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