---
title: "History of the metric system"
chunk: 8/10
source: "https://en.wikipedia.org/wiki/History_of_the_metric_system"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T03:39:48.437355+00:00"
instance: "kb-cron"
---

== Working draft of SI: Practical system of units ==
The 9th CGPM met in 1948, fifteen years after the 8th CGPM. 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. The CIPM's draft proposal was an extensive revision and simplification of the metric unit definitions, symbols, and terminology based on the MKS system of units.
Following astronomical observations, the second was set as a fraction of the year 1900. The electromagnetic base unit, as required by Giorgi, was accepted as the ampere. After negotiations with the CIS and IUPAP, two additional units—the degree kelvin and the candela—were also proposed as base units. For the first time, the CGPM made recommendations concerning derived units. At the same time, the CGPM adopted conventions for the writing and printing of unit symbols and numbers and catalogued the symbols for the most important MKS and CGS units of measure.

=== Time ===
Until the advent of the atomic clock, the most reliable timekeeper available to humanity 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. During the 20th century, it became apparent that the Earth's rotation was slowing down, resulting in days becoming 1.4 milliseconds longer each century—this was verified by comparing the calculated timings of eclipses of the Sun with those observed in antiquity going back to Chinese records of 763 BC. In 1956, the 10th CGPM instructed the CIPM to prepare a definition of the second; in 1958, the definition was published stating that the second (called an ephemeris second) would be calculated by extrapolation using Earth's rotational speed in 1900.

=== Electrical unit ===
Per Giorgi's proposals of 1901, 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 1946, 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. The definitions for the absolute electrical system, based on the ampere, were formalised in 1948. The draft proposed units with these names are very close, but not identical, to the international units.

=== Temperature ===
In the Celsius scale from the 18th century, temperature was expressed in degrees Celsius with the definition that ice melted at 0 °C and (at standard atmospheric pressure) water boiled at 100 °C. A series of lookup tables defined temperature in terms of interrelated empirical measurements made using various devices. In 1948, definitions relating to temperature had to be clarified. (The degree, as an angular measure, was adopted for general use in many countries, so, in 1948, the General Conference on Weights and Measures (CGPM) recommended that the degree centigrade, as used for the measurement of temperature, be renamed the degree Celsius.)
At the 9th CGPM, the centigrade temperature scale was renamed the Celsius scale, and the scale itself was fixed by defining the triple point of water as 0.01 °C, though the CGPM left the formal definition of absolute zero until the 10th CGPM when the name "Kelvin" was assigned to the absolute temperature scale, and the triple point of water was defined as being 273.16 °K.

=== Luminosity ===
Before 1937, the International Commission on Illumination (CIE from its French title, the Commission Internationale de l'Eclairage), in conjunction with the CIPM, produced a standard for luminous intensity to replace the various national standards. 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", was ratified by the CGPM in 1948.

=== Derived units ===
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.

== International System of Units (SI) ==

In 1952, the CIPM proposed the use of wavelength of a specific light source as the standard for defining length, and, in 1960, 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.
In 1960, Giorgi's proposals were adopted as the basis of the Système International d'Unités (International System of Units), the SI. This initial definition of the SI included six base units, the metre, kilogram, second, ampere, degree Kelvin, and candela, and sixteen coherent derived units.

== Evolution of the modern SI ==
The evolution of the SI after its publication in 1960 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.