Scien tists have adopted the metric system to simplify their calculations and promote communication across national boundaries. However, there have been two ideas as to which metric units should be preferred in science. Scientists working in laboratories, dealing with small quantities and distances, preferred to measure distance in centimeters and mass in grams. Scientists and engineers working in larger contexts preferred larger units: meters for distance and kilograms for mass. Everyone agreed that units of other quantities such as force, pressure, work, power, and so on should be related in a simple way to the basic units, but which basic units should be used?
The result was two clusterings of metric units in science and engineering. One cluster, based on the centimeter, the gram, and the second, is called the CGS system. The other, based on the meter, kilogram, and second, is called the MKS system.
When we say, for example, that the dyne is the CGS unit of force, this determines its definition: it is the force which accelerates a mass of one gram at the rate of one centimeter per second per second. The MKS unit of force, the newton, is the force which accelerates a mass of one kilogram at the rate of one meter per second per second. The ratio between a CGS unit and the corresponding MKS unit is usually a power of 10. A newton accelerates a mass 1000 times greater than a dyne does, and it does so at a rate 100 times greater, so there are 100 000 = 105 dynes in a newton.
The CGS system was introduced formally by the British Association for the Advancement of Science in 1874. It found almost immediate favor with working scientists, and it was the system most commonly used in scientific work for many years. Meanwhile, the further development of the metric system was based on meter and kilogram standards created and distributed in 1889 by the International Bureau of Weights and Measures (BIPM). During the 20th century, metric units based on the meter and kilogram--the MKS units--were used more and more in commercial transactions, engineering, and other practical areas. By 1950 there was some discomfort among users of metric units, because the need to translate between CGS and MKS units went against the metric ideal of a universal measuring system. In other words, a choice needed to be made.
In 1954, the Tenth General Conference on Weights and Measures (CGPM) adopted the meter, kilogram, second, ampere, degree Kelvin, and candela as the basic units for all international weights and measures, and in 1960 the Eleventh General Conference adopted the name International System of Units (SI) for this collection of units. (The "degree Kelvin" became the kelvin in 1967.) In effect, these decisions gave the central core of the MKS system preference over the CGS system. Although some of the CGS units remain in use for a variety of purposes, they are being replaced gradually by the SI units selected from the MKS system.
Following is a table of CGS units with their SI equivalents. Note that in some cases there is more than one name for the same unit. The CGS electromagnetic and electrostatic units are not included in this table, except for those which have special names.
| CGS unit | measuring | SI equivalent |
| barye (ba) | pressure | 0.1 pascal (Pa) |
| biot (Bi) | electric current | 10 amperes (A) |
| calorie (cal) | heat energy | 4.1868 joule (J) |
| darcy | permeability | 0.98692 x 10-12 square meter (m2) |
| debye (D) | electric dipole moment | 3.33564 x 10-30 coulomb meter (C·m) |
| dyne (dyn) | force | 10-5 newton (N) |
| emu | magnetic dipole moment | 0.001 ampere square meter (A·m2) |
| erg | work, energy | 10-7 joule (J) |
| franklin (Fr) | electric charge | 3.3356 x 10-10 coulomb (C) |
| galileo (Gal) | acceleration | 0.01 meter per second squared (m·s-2) |
| gauss (G) | magnetic flux density | 10-4 tesla (T) |
| gilbert (Gi) | magnetomotive force | 0.795 775 ampere-turns (A) |
| kayser (K) | wave number | 100 per meter (m-1) |
| lambert (Lb) | luminance | 3183.099 candelas per square meter (cd·m-2) |
| langley | heat transmission | 41.84 kilojoules per square meter (kJ·m-2) |
| line (li) | magnetic flux | 10-8 weber (Wb) |
| maxwell (Mx) | magnetic flux | 10-8 weber (Wb) |
| oersted (Oe) | magnetic field strength |
|
| phot (ph) | illumination | 104 lux (lx) |
| poise (P) | dynamic viscosity | 0.1 pascal second (Pa·s) |
| stilb (sb) | luminance | 104 candelas per square meter (cd·m-2) |
| stokes (St) | kinematic viscosity | 10-4 square meters per second (m2·s-1) |
| unit pole | magnetic flux | 1.256 637 x 10-7 weber (Wb) |
Nguồn: http://www.unc.edu/~rowlett/units/cgsmks.html
The seven SI base units
| Name | Symbol | Measure | Current (2005) formal definition[1] | Historical origin / justification | Dimension symbol |
|---|---|---|---|---|---|
| metre | m | length | "The metre is the length of the path travelled by light in vacuum during a time interval of 1 ⁄ 299792458 of a second." 17th CGPM (1983, Resolution 1, CR, 97) | 1 ⁄ 10,000,000 of the distance from the Earth's equator to the North Pole measured on the circumference throughParis. | L |
| kilogram | kg | mass | "The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram." 3rd CGPM (1901, CR, 70) | The mass of one litre of water. A litre is one thousandth of a cubic metre. | M |
| second | s | time | "The second is the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom." 13th CGPM (1967/68, Resolution 1; CR, 103) "This definition refers to a caesium atom at rest at a temperature of 0 K." (Added by CIPM in 1997) | The day is divided in 24 hours, each hour divided in 60 minutes, each minute divided in 60 seconds. A second is 1 ⁄ (24 × 60 × 60) of the day | T |
| ampere | A | electric current | "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 metre apart in vacuum, would produce between these conductors a force equal to 2 × 10−7 newton per metre of length." 9th CGPM (1948) | The original "International Ampere" was defined electrochemically as the current required to deposit 1.118 milligrams of silver per second from a solution of silver nitrate. Compared to the SI ampere, the difference is 0.015%. | I |
| kelvin | K | thermodynamic temperature | "The kelvin, unit of thermodynamic temperature, is the fraction 1 ⁄ 273.16of the thermodynamic temperature of the triple point of water." 13th CGPM (1967/68, Resolution 4; CR, 104) "This definition refers to water having the isotopic composition defined exactly by the following amount of substance ratios: 0.000 155 76 mole of 2H per mole of 1H, 0.000 379 9 mole of 17O per mole of 16O, and0.002 005 2 mole of 18O per mole of 16O." (Added by CIPM in 2005) | The Celsius scale: the Kelvin scale uses the degree Celsius for its unit increment, but is a thermodynamic scale (0 K is absolute zero). | Θ |
| mole | mol | amount of substance | "1. 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.'
2. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles."
14th CGPM (1971, Resolution 3; CR, 78) "In this definition, it is understood that unbound atoms of carbon 12, at rest and in their ground state, are referred to." (Added by CIPM in 1980) | Atomic weight or molecular weight divided by the molar mass constant, 1 g/mol. | N |
| candela | cd | luminous intensity | "The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian." 16th CGPM (1979, Resolution 3; CR, 100) | The candlepower, which is based on the light emitted from a burning candle of standard properties. | J |
| 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. Examples of such SI derived units are given in Table 2, where it should be noted that the symbol 1 for quantities of dimension 1 such as mass fraction is generally omitted.
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 | For ease of understanding and convenience, 22 SI derived units have been given special names and symbols, as shown in Table 3.
 t and the thermodynamic temperature difference T between the melting point of gallium and the triple point of water may be written as t = 29.7546 °C = T = 29.7546 K.
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