MKS is the system of units based on measuring lengths in meters, mass in kilograms, and time in seconds. MKS is generally used in engineering and beginning physics, where the so-called cgs system (based on the centimeter, gram, and second) is commonly used in theoretic physics. The most familiar units of electricity and magnetism (ohm, farad, coulomb, etc.) are MKS units.
CGS is the system of units based on measuring lengths in centimeters, mass in grams, and time in seconds. It is a metric system, although not the flavor of the metric system used most commonly. It was introduced by the British Association for the Advancement of Science in 1874, and was immediately adopted by many working scientists.
There are several flavors of the cgs system: "cgs electrostatic," "cgs electromagnetic," and "cgs Gaussian." None of these are part of the SI system, except for units such as the centimeter defined in both systems (Taylor 1995, p. 11). The cgs Gaussian system is nonetheless commonly used in theoretical physics, while the MKS system (based on the meter, kilogram, and second) is commonly used in engineering and physics instruction.
"SI" stands for "System International" and is the set of physical units agreed upon by international convention. The SI units are sometimes also known as MKS units, where MKS stands for "meter, kilogram, and second." In 1939, the CCE recommended the adoption of a system of units based on the meter, kilogram, second, and ampere. This proposal was approved by the Comité International des Poids et Mesures (CIPM) in 1946. Following an international inquiry by the Bureau International des Poids et Mesures (BIPM), which began in 1948, in 1954 the 10th Conférence Générale des Poids et Mesures (CGPM) approved the introduction of the ampere, kelvin, and candela as base units for electric current, thermodynamic temperature, and luminous intensity, respectively. However, the ampere is scheduled to be phased out as a base unit in the near future in favor of the ohm, which can be measured extremely accurately using the quantum Hall effect. In turn, the volt will probably replace the ohm further in the future when measurements using Josephson junctions increase in precision.
The name International System of Units (SI) was given to the system by the 11th CGPM in 1960. At the 14th CGPM in 1971, the current version of the SI was completed by adding the mole as base unit for amount of substance, bringing the total number of base units to seven. The seven fundamental units are summarized in the following table.
CGS is the system of units based on measuring lengths in centimeters, mass in grams, and time in seconds. It is a metric system, although not the flavor of the metric system used most commonly. It was introduced by the British Association for the Advancement of Science in 1874, and was immediately adopted by many working scientists.
There are several flavors of the cgs system: "cgs electrostatic," "cgs electromagnetic," and "cgs Gaussian." None of these are part of the SI system, except for units such as the centimeter defined in both systems (Taylor 1995, p. 11). The cgs Gaussian system is nonetheless commonly used in theoretical physics, while the MKS system (based on the meter, kilogram, and second) is commonly used in engineering and physics instruction.
| unit | symbol | MKS (abbrev.) | cgs (abbrev.) |
| acceleration | a | m s | Gal |
| capacitance | C | Farad (F) | cm |
| charge | q | Coulomb (C) | esu |
| current | I | Ampere (A) | esu s-1 |
| electric field | E | V m-1 | statvolt cm-1 |
| electric potential | V,  | Volt (V) | statvolt |
| energy, work | E, W | Joule (J) | erg |
| force | F | Newton (N) | dyne |
| inductance | L | Henry (H) | cm-1 s |
| length | l, d | meter (m) | centimeter (cm) |
| magnetic field | B | Tesla (T) | Gauss (G) |
| magnetic flux |  | Weber (w) | Gauss cm |
| mass | m | kilogram (kg) | gram (g) |
| momentum | p | kg m s-1 | g cm s-1 |
| power | P | Watt (W) | erg s-1 |
| pressure | P | Pascal (Pa) | bar |
| resistance | R | Ohm ( ) | cm-1 s |
| temperature | T | Kelvin (K) | Kelvin (K) |
| time | t | second (s) | second (s) |
| velocity | v | m s-1 | cm s-1 |
"SI" stands for "System International" and is the set of physical units agreed upon by international convention. The SI units are sometimes also known as MKS units, where MKS stands for "meter, kilogram, and second." In 1939, the CCE recommended the adoption of a system of units based on the meter, kilogram, second, and ampere. This proposal was approved by the Comité International des Poids et Mesures (CIPM) in 1946. Following an international inquiry by the Bureau International des Poids et Mesures (BIPM), which began in 1948, in 1954 the 10th Conférence Générale des Poids et Mesures (CGPM) approved the introduction of the ampere, kelvin, and candela as base units for electric current, thermodynamic temperature, and luminous intensity, respectively. However, the ampere is scheduled to be phased out as a base unit in the near future in favor of the ohm, which can be measured extremely accurately using the quantum Hall effect. In turn, the volt will probably replace the ohm further in the future when measurements using Josephson junctions increase in precision.
The name International System of Units (SI) was given to the system by the 11th CGPM in 1960. At the 14th CGPM in 1971, the current version of the SI was completed by adding the mole as base unit for amount of substance, bringing the total number of base units to seven. The seven fundamental units are summarized in the following table.
| physical quantity | symbol | unit abbreviation | unit name |
| length | l | m | meter |
| mass | m | kg | kilogram |
| time | t | s | second |
| current | I | A | Ampere |
| temperature | T | K | Kelvin |
| luminous intensity | cd | candela | |
| amount of substance | n | mol | mole |
The derived SI units consist of combinations of the seven base units, and are summarized in the following table.
| quantity | symbol | SI symbol | SI unit |
| area | A |  | square meter |
| volume | V |  | cubic meter |
| plane angle |  | rad | radian |
| solid angle |  | sterrad | steradian |
| frequency | f | Hz | Hertz |
| velocity | v |  | meters per second |
| acceleration | a |  | meters per second squared |
| force | F | N | Newton |
| pressure | P or p | Pa | Pascal |
| power | P | W | Watt |
| energy | E | J | Joule |
| voltage | V | V | Volt |
| resistance | R | | Ohm |
| conductance | G | S | Siemens |
| charge | Q | C | Coulomb |
| capacitance | C | F | Farad |
| magnetic flux |  | Wb | Weber |
| magnetic flux density | B | T | Tesla |
| inductance | L | H | Henry |
| luminous flux | F | lm | lumen |
| illumination | E | lx | lux |
| activity | A | Bq | Becquerel |
| energy dose | Gy | Gray | |
| equivalent dose | Sv | Sievert |
In 1960, the 11th CGPM adopted a first series of prefixes and symbols of prefixes to form the names and symbols of decimal multiples and submultiples of SI units. Over the years, the list has been extended as summarized in the following table.
| factor | prefix | symbol |
| 1024 | yotta- | Y |
| 1021 | zetta- | Z |
| 1018 | exa- | E |
| 1015 | peta- | P |
| 1012 | tera- | T |
| 109 | giga- | G |
| 106 | mega- | M |
| 103 | kilo- | k |
| 102 | hecto- | h |
| 101 | deca- | da |
 | deci- | d |
 | centi- | c |
 | milli- | m |
 | micro- |  |
 | nano- | n |
 | pico- | p |
 | femto- | f |
 | atto- | a |
 | zepto- | z |
 | yocto- | y |
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