Brief history of the International System

The creation of the decimal Metric System at the time of the French Revolution and the subsequent deposition of two platinum standards representing the meter and the kilogram, on 22 June 1799, in the Archives de la République in Paris can be seen as the first step in the development of the present International System of Units.

In 1832, Gauss strongly promoted the application of this Metric System as a coherent system of units for the physical sciences. 

In 1874 the British Association for the Advancement of Science (BAAS) introduced the CGS system, a three-dimensional coherent unit system based on the three mechanical units centimeter, gram and second, using prefixes ranging from micro to mega to express decimal submultiples and multiples. The following development of physics as an experimental science was largely based on this system.

After the establishment of the Meter Convention on May, 20 1875 the International Committee for Weights and Measures (CIPM, from the French Comité international des poids et mesures) concentrated on the construction of new prototypes taking the meter and kilogram as the base units of length and mass.

The name International System of Units (SI) was given to the system in 1960.

The SI is founded on seven SI base units for seven base quantities assumed to be mutually independent.

SI derived units

Other quantities, called derived quantities, are defined in terms of the seven base quantities via a system of quantity equations. The SI derived units for these derived quantities are obtained from these equations and the seven SI base units.

Definitions of the SI base Units


Definition. The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

Unit of length (meter)

The origins of the meter go back to at least the 18th century. At that time, there were two competing approaches to the definition of a standard unit of length. In 1791, soon after the French Revolution, the French Academy of Sciences chose the meridian definition over the pendulum definition because the force of gravity varies slightly over the surface of the earth, affecting the period of the pendulum.

Thus, the meter was intended to equal 10-7 or one ten-millionth of the length of the meridian through Paris from pole to the equator.

In 1889, a new international prototype was made of an alloy of platinum with 10 percent iridium, to within 0.0001, that was to be measured at the melting point of ice.

The 1889 definition of the meter was replaced by the definition based upon a wavelength of krypton-86 radiation.


Definition. The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.

Unit of mass (kilogram)

At the end of the 18th century, a kilogram was the mass of a cubic decimeter of water. In 1889, was built the international prototype of the kilogram, made of platinum-iridium, and declared: This prototype shall henceforth be considered to be the unit of mass.


Definition. The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.

Unit of time (second)

The unit of time, the second, was defined originally as the fraction 1/86 400 of the mean solar day. The exact definition of “mean solar day” was left to astronomical theories. However, measurement showed that irregularities in the rotation of the Earth could not be taken into account by the theory and have the effect that this definition does not allow the required accuracy to be achieved.

Experimental work had, however, already shown that an atomic standard of time-interval, based on a transition between two energy levels of an atom or a molecule, could be realized and reproduced much more precisely. Considering that a very precise definition of the unit of time is indispensable for the International System, the given definition of the second was replaced by the following: “the second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom”.

Electric current

Definition. The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 x 10-7 newton per meter of length.

Unit of electric current (ampere)

Electric units, called “international,” for current and resistance were introduced by the International Electrical Congress held in Chicago in 1893, and the definitions of the “international” ampere and the “international” ohm were confirmed by the International Conference of London in 1908.


Definition. The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

Unit of thermodynamic temperature (kelvin)

The definition of the unit of thermodynamic temperature was given in 1954. In this year was assigned the temperature 273.16 K to the triple point of water.  In 1967 was adopted the name kelvin (symbol K) instead of “degree Kelvin” (symbol °K) and defined the unit of thermodynamic temperature as follows: “the kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water”.

Because of the way temperature scales used to be defined, it remains common practice to express thermodynamic temperature, symbol T, in terms of its difference from the reference temperature T0 = 273.15 K, the ice point. This temperature difference is called a Celsius temperature, symbol t, and is defined by the quantity equation

The unit of Celsius temperature is the degree Celsius, symbol °C, which is by definition equal in magnitude to the kelvin. A difference or interval of temperature may be expressed in kelvins or in degrees Celsius . The numerical value of a Celsius temperature t expressed in degrees Celsius is given by

t/°C = T/K – 273.15

Amount of substance

Definition. The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12; its symbol is “mol.”

Unit of amount of substance (mole)

Following the discovery of the fundamental laws of chemistry, units called, for example, “gram-atom” and “gram-molecule,” were used to specify amounts of chemical elements or compounds. These units had a direct connection with “atomic weights” and “molecular weights,” which were in fact relative masses. “Atomic weights” were originally referred to the atomic weight of oxygen, by general agreement taken as 16. But whereas physicists separated isotopes in the mass spectrograph and attributed the value 16 to one of the isotopes of oxygen, chemists attributed that same value to the (slightly variable) mixture of isotopes 16, 17, and 18, which was for them the naturally occurring element oxygen. Finally, an agreement between the International Union of Pure and Applied Physics (IUPAP) and the International Union of Pure and Applied Chemistry (IUPAC) brought this duality to an end in 1959/60. Physicists and chemists have ever since agreed to assign the value 12, exactly, to the “atomic weight,” correctly the relative atomic mass, of the isotope of carbon with mass number 12 (carbon 12, 12C). The unified scale thus obtained gives values of relative atomic mass.

It remained to define the unit of amount of substance by fixing the corresponding mass of carbon 12; by international agreement, this mass has been fixed at 0.012 kg, and the unit of the quantity “amount of substance” was given the name mole (symbol mol).

Luminous intensity

Definition. The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

Unit of luminous intensity (candela)

Originally, each country had its own, and rather poorly reproducible, unit of luminous intensity; it was necessary to wait until 1909 to see a beginning of unification on the international level, when the national laboratories of the United States of America, France, and Great Britain decided to adopt the international candle represented by carbon filament lamps.

The units of luminous intensity based on flame or incandescent filament standards in use in various countries before 1948 were replaced initially by the “new candle” based on the luminance of a Planckian radiator (a blackbody) at the temperature of freezing platinum. In 1948 was adopted a new international name for this unit, the candela (symbol cd).

In 1979, because of the experimental difficulties in realizing a Planck radiator at high temperatures and the new possibilities offered by radiometry, i.e., the measurement of optical radiation power, was adopted the current definition of the candela.

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