System of Units

System of Units

 

a set of fundamental and derived units that pertains to some system of quantities and is formed in accordance with accepted principles. A system of units is constructed on the basis of physical theories that reflect the interrelationship existing in nature between physical quantities. When the units of a system are defined, the physical relations used are arranged in a sequence such that each succeeding expression contains only one new quantity. This method permits the unit of a physical quantity to be defined in terms of previously defined units and, in the final analysis, in terms of the fundamental (independent) units of the system (seeUNITS, PHYSICAL).

The first systems of units employed units of length and mass as the fundamental units—for example, the foot and the English pound in Great Britain and the arshin and the funt (Russian pound) in Russia. These systems included multiple and fractional units that had their own names, such as the yard and inch in the English system and the sazhen’, vershok, and fut (foot) in the Russian system. Complicated sets of derived units were thus created. The difference in the national systems of units was an inconvenience for trade and industry. In response to this problem, the idea of the metric system arose in France in the 18th century. The metric system has been the basis for the international standardization of the units of length (the meter), mass (the kilogram), and the principal derived units—the units of area, volume and density.

In the 19th century K. F. Gauss and W. E. Weber proposed a system of units for electrical and magnetic quantities that was called absolute by Gauss. The fundamental units of the system were the millimeter, the milligram, and the second. The derived units were defined by equations expressing the relations between the quantities in simplest form—that is, the numerical coefficients were equal to unity. Systems defined in this way were later called coherent.

In the second half of the 19th century the British Association for the Advancement of Science adopted two systems of units: the centimeter-gram-second (cgs) electrostatic system and the cgs electromagnetic system (see). This act stimulated the creation of other systems of units, for example, the Gaussian system, the meter-kilogram-force system, and the meter-ton-second system. The Italian physicist G. Giorgi proposed in 1901 a system of units based on the meter, the kilogram, the second, and one electrical unit; the ampere was subsequently chosen as the electrical unit (seeMETER-KILOGRAM-SECOND-AMPERE SYSTEM). The Giorgi system included such widely used practical units as the ampere, volt, ohm, watt, joule, farad, and henry. The Giorgi system formed the basis for the International System of Units (SI), which was approved by the Eleventh General Conference of Weights and Measures in 1960. The International System has seven base units: the meter, kilogram, second, ampere, kelvin, mole, and candela. The creation of the system improved the outlook for a general standardization of units. Many countries decided to adopt the system or to give it preference.

Besides the practical systems of units, physics makes use of systems that are based on universal physical constants, such as the speed of propagation of light in a vacuum, the charge of an electron, and Planck’s constant (seeNATURAL SYSTEMS OF UNITS).

REFERENCES

Burdun, G. D. Edinitsy fizicheskikh velichin, 4th ed. Moscow, 1967.
Burdun, G. D. Spravochnikpo Mezhdunarodnoi sisteme edinits. Moscow, 1971.
Burdun, G. D., and B. N. Markov. Osnovy metrologii. Moscow, 1972.

K. P. SHIROKOV

References in periodicals archive ?
assignments, dissertations and manuscripts, where the International System of Units (SI) is not used optimally.
DUBAI - Ajman Lands and Properties Department, or ALPD, and Ajman Real Estate Regulatory Agency (ARRA), said on Wednesday that they have embarked on implementing the use of the International System of Units (SI) replacing feet with metre.
Dubai The UAE will implement the International System of Units (SI) replacing feet, inches and yards in favour of metres, on November 11.
Regarding units, the International System of Units (SI) is mandated in this and all NIST publications, so you will of course see units of the SI system such as the meter (in), kilogram (kg), second (s), etc.
30 that it had lost the $125 million Mars Climate Orbiter because the force exerted by the orbiter's thrusters remained in the system of units based on pounds and feet rather than being converted to metric.
The 2001 Edition of NIST Special Publication 330, The International System of Units (SI), is now available.
Redefining the kilogram represents more of a tidying up of the international system of units (SI) than a response to a need for an improved mass standard.
A new paper from a NIST researcher describes the essential physics of SET devices, discusses various schemes for making capacitance and current standards based on SET devices, and covers the relevance of SET standards for fundamental constants and the International System of Units. The development of a NIST prototype SET capacitance standard is described as proceeding along three pathways: (1) a transportable version of the prototype is being constructed for direct comparison with the calculable capacitor at NIST headquarters in Gaithersburg, MD; (2) a detailed uncertainty analysis of all aspects of the standard is being developed; and (3) an effort is under way to design an easy-to-use, robust and automated system with computer control of as many functions as possible.
Mass stands out as the last physical parameter of the International System of Units that still rests on a specific artifact.
In soliciting contributions to the Centennial Issue from the NIST staff, I took into account the fact that metrology requires a system of units in terms of which measurement results can be expressed.

Full browser ?