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Defining the International System of Units (SI)

Defining the International System of Units (SI)
Outer circle has one wedge for each of the 7 SI units (kilogram, meter, second, ampere, kelvin, mole, and candela) and the inner circle has wedges for the 7 important constants.
Download the image files for this logo.
Credit: BIPM

Early weights and measures began out of the need for civilizations to understand the complexities of their environment and to create a form of commerce. Archeological evidence shows human beings used available resources to conduct their business. Length was based on the human anatomy; available items found in nature, such as seeds, were used to measure weight; and time was measured by the movement of the sun and phases of the moon.

Different civilizations around the world employed their individual measurement systems, but with the growth of world trade, the inconsistencies of the various systems became problematic. Thus, the search for a precise and consistent system began.

With the establishment of the Bureau International des Poids et Mesures, or International Bureau of Weights and Measures (BIPM) at Versailles on May 20, 1875, measurement standards began to be officially defined and standard prototypes were distributed throughout world.

Over time, advances in technology have resulted in more precise measurements, generating an evolution of definitions for the Système Internationale, or International System of Units (SI). These units are known as the meter (m), kilogram (kg), second (s), kelvin (K), candela (cd), mole (mol), and ampere (A).


Measurement of Length

1799: The meter is one ten-millionth part of the quadrant of the earth based on a measurement of a meridian between Dunkirk, France and Barcelona, Spain, and represented by a platinum bar where the distance between the polished parallel ends is a meter. 

1889: The meter is the distance between the two graduation lines at 0 oC on the International Prototype Meter.

1960: The meter is 1,650,763.73 vacuum wavelengths of the krypton isotope having an atomic weight of 86.

1983: The meter is the length of the path traveled by light in a vacuum during a time interval of 1/299,792,458 of a second.

Committee Meter
Committee Meter



This standard was made in France in 1799 and used by the U.S. Coast and Geodetic Survey as a length standard for scientific work from 1807-1893.
Credit: NIST Museum Collection
Meter Bar 27
International Prototype Meter Bar No. 27 
Krypton-86 Lamp
Krypton-86 Lamp, circa 1960. This Krypton-86 lamp replaced the National Prototype Meter Bar No. 27 in 1960 as the U.S. primary standard for length. Wavelengths of light emitted by electrically excited atoms of the gas Krypton-86 defined the meter from 1960-1983.
Credit: NIST Museum Collection
Iodine Stabilized Laser
Iodine Stabilized Laser.

National Bureau of Standards.

Helium neon laser tube.

circa late 1970s or 1980.

Precision length measurements are made through laser interferometry, which determines a physical length as a number of wavelengths. One meter is approximately 1 579 800.762 wavelengths of the iodine stabilized laser. The wavelength of a laser is related to its frequency. Frequency is measured in terms of time (cycles per second), creating an exact relationship between time and length. This laser was commercially built based on a design by Howard Layer of the National Bureau of Standards (NBS). It was one of the earliest lasers used by NBS as a standard for length.
Credit: NIST Museum Collection


Measurement of Weight

18th Century: The kilogram is equal to the mass of a cubic decimeter of water.

1889: The kilogram is equal to the mass of the International Prototype of the kilogram.

2019: The kilogram is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10−34 when expressed in the unit J s, which is equal to kg m² s−1, where the meter and the second are defined in terms of c and ΔvCs.

Kilogram and Liter Standard
Kilogram and Liter Standard, Liquid Mass

Maker: Louis A. Fischer National Bureau of Standards Gold-plated brass, circa 1905.

This airtight hollow sphere has a volume of one cubic decimeter. When filled with water, its mass equaled one kilogram. It was constructed to replace an earlier liquid mass standard made in 1844 by Joseph Saxton of the U.S. Coast Survey and which was suspected of leaking air.
Credit: NIST Museum Collection
Kilogram No. 20 standard of mass at the National Bureau of Standards
The platinum-iridium cylinder (right) was the primary standard kilogram for all metric measurements in the U. S. Known as Kilogram No. 20, it was a copy of the International Prototype Kilogram, which was preserved at the International Bureau of Weights and Measures at Sevres, France. c. 1875
Credit: NIST Museum Collection
NIST-4 Kibble Balance Model
Model of the NIST-4 Kibble Balance used to redefine the unit of mass, kilogram, 2020.
Credit: NIST Museum Collection


Measurement of Time

Unknown Date: The second is the fraction 1/86,400 of the mean solar day.

1967: 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.

Tow large metal boxes with a wood box in between, then space, then two more metal boxes with wood box inbetween and inside of that is the inside of a quartz time standard
Quartz Crystal Resonator, circa 20th century. Realization of the second using mechanically swinging clock pendulums was substituted in 1929 by the natural

vibrations of electrically stimulated quartz. Quartz crystals vibrating at a naturally

consistent rate (millions of times per second) were used as accurate “pendulums” in

electronic timepieces.
Credit: NIST
Deflecting Magnets from NBS-II Atomic Clock
These deflecting magnets were part of the National Bureau of Standards Atomic Clock II, which was the United States official frequency standard from 1960-1963. The magnets deflected a beam of cesium atoms from an oven source into the microwave interrogation cavity, or Ramsey cavity, of the atomic clock, where the frequency of the cesium atoms could be detected.
Credit: NIST Museum Collection


Measurement of Temperature

1954: The kelvin, the unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

2019: The kelvin is defined by taking the fixed numerical value of the Boltzmann constant k to be 1.380 649 × 10–23 when expressed in the unit J K–1, which is equal to kg m2 s–2 K–1, where the kilogram, meter and second are defined in terms of h, c and ΔvCs.

Triple-Point Cell
Triple-Point Cell. National Bureau of Standards, 1968. The triple-point is a unique temperature at which water can exist in its solid, liquid and gas phases all in equilibrium. It is assigned the value of 273.16 Kelvins (0.01 degrees Celsius). The triple-point is realized using the Triple-Point Cell, parts of which are alternatively frozen and thawed to achieve the triple-point.
Credit: NIST Museum Collection
Standard Platinum Resistance Thermometer
Standard Platinum Resistance Thermometer. This device, whose electrical resistance changes with temperature, is used to realize the International Practical Temperature Scale between 13.8033 K (-259.3467 oC) and 1234.93 K (961.78 oC).
Credit: NIST Museum Collection
Freezing-Point Cell
Freezing-Point Cell, Aluminum. A Standard Platinum Resistance Thermometer is calibrated by immersing it sequentially into a series of fixed-point cells. The aluminum freezing-point cell has a freezing point defined as 660.323 oC on the International Temperature Scale.
Credit: NIST Museum Collection


Measurement of Light

1948: The candela is the luminance of a Planck radiator (a black body) at the temperature of freezing platinum.

1979: 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.

2019: The candela is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz, Kcd, to be 683 when expressed in the unit lm W–1, which is equal to cd sr W–1, or cd sr kg–1 m–2 s3, where the kilogram, meter and second are defined in terms of h, c and ΔvCs.

what looks to be an old fashioned light bulb
Candela Standard Lamp, 1948. A 500-watt gas-filled incandescent lamp, the Candela Standard Lamp was used as the luminous intensity standard from 1948-1979. The lamp has a luminous intensity of approximately 790 candela. It produced a visible green/yellow, the color the human eye responds to most strongly.
Credit: NIST
three pictures of a device that looks like an old camera
Photodetector for Standard Candela, circa 1979. This silicon diode photodetector included a filter that spectrally matched the accepted human visual response. This photodetector was one of the first detector devices used to realize the candela standard for the United States.
Credit: NIST

MOLE (mol)

Measurement of Mole

1971: 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.

2019: The mole is defined equal to 6.022 140 76 x 1023 molecular entities.

Mole Cube Samples
Mole Cube Samples, National Institute of Standards and Technology, Aluminum, copper and carbon graphite. The mole is an amount of a substance. All moles equal 6.022 140 76 x 1023 atoms. One mole can weighdifferent amounts based on the mass of the particular atoms, i.e., aluminum weighs 26.98 grams; copper 63.55 grams; and carbon 12.01 grams.
Credit: NIST Museum Collection
Interferometer for Determining Mole
Interferometer for Determining Mole. Maker: Albert Henins, National Bureau of Standards, circa 1970s. This three-blade silicon single-crystal interferometer, used with an x-ray source and an iodine laser, was used to determine how many atoms and their spacing in a given volume of a material. From that measure a calculation of the total number of atoms could be determined.
Credit: NIST Museum Collection


Measurement of Electricity

1948: 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.

2019: The ampere is defined by taking the fixed numerical value of the elementary charge e to be 1.602 176 634 × 10–19 when expressed in the unit C, which is equal to A s, where the second is defined in terms of ΔvCs.

Kelvin Ampere Balance
Kelvin Ampere Balance, Inventor(s): Lord Kelvin and James White Ltd. 1902. The Kelvin balance compares the magnetic force between two wire coils to the mechanical force generated by metal weights on a balance arm. Once the coils and balance arm are in equilibrium, the current can be calculated from the amount of weight placed on the balance arm.
Credit: NIST Museum Collection

Defining the International System of Units (SI) exhibit image
Photograph of the physical exhibit is located in the NIST Museum Main Room on the Gaithersburg, MD. campus.
Credit: Sarah Reeves/NIST Museum

Example of how to reference this exhibit:
NIST Museum. 2023. Defining the International System of Units (SI). Gaithersburg, MD: National Institute of Standards and Technology. Online.

Author. Year. Exhibit Name. Place published: Publisher. Online. URL.




Created September 29, 2023, Updated October 30, 2023