Machinery's Handbook, 31st Edition
Metric Systems
2831
MEASURING UNITS Metric Systems of Measurement
A metric system of measurement was first established in France in the years following the French Revolution, and various systems of metric units have been developed since that time. All metric unit systems are based, at least in part, on the International Metric Standards, which are the meter and kilogram, or decimal multiples or submultiples of these standards. In 1795, a metric system called the centimeter-gram-second (cgs) system was pro- posed, and it was adopted in France in 1799. In 1873, the British Association for the Advancement of Science recommended the use of the cgs system, and since then it has been widely used in all branches of science throughout the world. From the base units in the cgs system are derived the following: Unit of velocity = 1 centimeter per second Acceleration due to gravity (at Paris) = 981 centimeters per second per second Unit of force = 1 dyne = 1 ∕ 981 gram Unit of work = 1 erg = 1 dyne-centimeter Unit of power = 1 watt = 10,000,000 ergs per second Another metric system, called the MKS (meter-kilogram-second) system of units, was proposed by Professor G. Giorgi in 1902. In 1935, the International Electrotechnical Commission (IEC) accepted his recommendation that this system of units of mechanics be linked with the electromagnetic units by the adoption of a fourth base unit. In 1950, the IEC adopted the ampere, the unit of electric current, as the fourth unit, and the MKSA system thus came into being. A gravitational system of metric units, known as the technical system, is based on the meter, the kilogram as a force, and the second. It has been widely used in engineering. Because the standard of force is defined as the weight of the mass of the standard kilogram, the fundamental unit of force varies due to the difference in gravitational pull at different locations around the earth. By international agreement, a standard value for acceleration due to gravity was chosen (9.81 meters per second squared) that for all practical measurements is approximately the same as the local value at the point of measurement. International System of Units (SI).—The Conférence Générale des Poids et Mesures (CGPM) is the body responsible for all international matters concerning the metric system, which was originally based on the four MKSA units for: length ( meter ), time ( second ), mass ( kilogram ), and electric current ( ampere ). In 1954, a unit for temperature ( kelvin ) and another for luminous intensity ( candela ) were added. Over the years, these standards have been refined and redefined. In 1960, the CGPM formally renamed the metric system the Système International d’Unites (abbreviated SI in all languages). In 1971, the 14th CGPM adopted a seventh base unit, the mole , for the amount of substance in a physical sample. In 2011, CGPM stated its intention of defining all seven base units with “the explicit-constant formulation,” where all units would be defined indirectly through their relation to a fundamental physical constant, as had been done for the speed of light. For several more years, only three of the seven base units — the meter, candela, and second—were as yet defined in terms of fundamental physi- cal constants. But in May 2019, the other four base units—the mole, ampere, kelvin, and kilogram—were redefined, no longer based on physical objects, but on universal physical constants.
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