International System Of Electrical And Magnetic Units
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The International System of Electrical and Magnetic Units is an obsolete system of units used for measuring electrical and magnetic quantities. It was proposed as a system of practical international units (e.g., the international ampere, the international ohm, the international volt) by unanimous recommendation at the International Electrical Congress (Chicago, 1893), discussed at other Congresses, and finally adopted at the International Conference on Electric Units and Standards in London in 1908. It was rendered obsolete by the inclusion of electromagnetic units in the
International System of Units The International System of Units, internationally known by the abbreviation SI (from French ), is the modern form of the metric system and the world's most widely used system of measurement. It is the only system of measurement with official s ...
(SI) at the 9th General Conference on Weights and Measures in 1948.


Earlier systems

The link between electromagnetic units and the more familiar units of
length Length is a measure of distance. In the International System of Quantities, length is a quantity with Dimension (physical quantity), dimension distance. In most systems of measurement a Base unit (measurement), base unit for length is chosen, ...
,
mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...
and
time Time is the continuous progression of existence that occurs in an apparently irreversible process, irreversible succession from the past, through the present, and into the future. It is a component quantity of various measurements used to sequ ...
was first demonstrated by
Carl Friedrich Gauss Johann Carl Friedrich Gauss (; ; ; 30 April 177723 February 1855) was a German mathematician, astronomer, geodesist, and physicist, who contributed to many fields in mathematics and science. He was director of the Göttingen Observatory and ...
in 1832 with his measurement of the Earth's magnetic field, and the principle was extended to electrical measurements by Franz Ernst Neumann in 1845. A complete system of metric electrical and magnetic units was proposed by Wilhelm Eduard Weber in 1851, based on the idea that electrical units could be defined solely in relation to absolute units of length, mass, and time. Weber's original proposal was based on a millimetre–milligram–second system of units. The development of the electric telegraph (an invention of Gauss and Weber) demonstrated the need for accurate electrical measurements. At the behest of William Thomson, the
British Association for the Advancement of Science The British Science Association (BSA) is a Charitable organization, charity and learned society founded in 1831 to aid in the promotion and development of science. Until 2009 it was known as the British Association for the Advancement of Scienc ...
(B.A.) set up a committee in 1861, initially to examine standards for electrical resistance, which was expanded in 1862 to include other electrical standards. After two years of discussion, experiment and considerable differences of opinion, the committee decided to adapt Weber's approach to the CGS system of units, but used metre, gramme and second as their absolute units. However these units were both difficult to realize and (often) impractically small. To overcome these handicaps, the B.A. also proposed a set of "practical" or "reproduceable" units, which were not directly linked to the CGS system but which were, as near as experimental accuracy allowed, equal to multiples of the corresponding CGS units. The B.A. had developed ''two'' sets of CGS units. The practical units were based on the electromagnetic set of units rather than the electrostatic set.


1893 system

The B.A. system of practical units gained considerable international support, and was adopted – with one important modification – by the First International Conference of Electricians (Paris, 1881). The British Association had constructed an artefact representation of the ohm (a standard length of resistance wire which had a resistance of 109 CGS units of electric resistance, that is one ohm) whereas the international conference preferred a method of realization that could be repeated in different laboratories in different countries. The chosen method was based on the resistivity of mercury, by measuring the resistance of a column of mercury of specified dimensions (106 cm × 1 mm2): however, the chosen length of column was almost 3 millimetres too short, leading to a difference of 0.28% between the new practical units and the CGS units which were supposedly their basis. The anomaly was resolved at another international conference, in Chicago in 1893, by a correction in the definition of the ohm. The units agreed at this conference were termed "international" units, to distinguish them from their predecessors. The 1893 system had three base units: the international
ampere The ampere ( , ; symbol: A), often shortened to amp,SI supports only the use of symbols and deprecates the use of abbreviations for units. is the unit of electric current in the International System of Units (SI). One ampere is equal to 1 c ...
, the international ohm and the international
volt The volt (symbol: V) is the unit of electric potential, Voltage#Galvani potential vs. electrochemical potential, electric potential difference (voltage), and electromotive force in the International System of Units, International System of Uni ...
. The international units did not have the same formal legal status as the
metre The metre (or meter in US spelling; symbol: m) is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of of ...
and the
kilogram The kilogram (also spelled kilogramme) is the base unit of mass in the International System of Units (SI), equal to one thousand grams. It has the unit symbol kg. The word "kilogram" is formed from the combination of the metric prefix kilo- (m ...
through the
Metre Convention The Metre Convention (), also known as the Treaty of the Metre, is an international treaty that was signed in Paris on 20 May 1875 by representatives of 17 nations: Argentina, Austria-Hungary, Belgium, Brazil, Denmark, France, German Empire, Ge ...
(1875), although several countries adopted the definition within their national laws (e.g., the United States, through Public Law 105 of July 12, 1894).


Overdefinition and the 1908 modification

The 1893 system of units was overdefined, as can be seen from an examination of
Ohm's law Ohm's law states that the electric current through a Electrical conductor, conductor between two Node (circuits), points is directly Proportionality (mathematics), proportional to the voltage across the two points. Introducing the constant of ...
: : ''V'' = ''I'' ''R''. By Ohm's law, knowing any two of the physical quantities ''V'', ''I'' or ''R'' (potential difference, current or resistance) will define the third, and yet the 1893 system defines the units for all three quantities. With improvements in measurement techniques, it was soon recognised that :1 Vint ≠ 1 Aint × 1 Ωint. The solution came at an international conference in London in 1908. The essential point was to reduce the number of base units from three to two by redefining the international volt as a derived unit. There were several other modifications of less practical importance: *the international ampere and the international ohm were formally defined in terms of the corresponding CGS electromagnetic units, with the 1893 definitions retained as preferred realizations; *the preferred realization of the international volt was in terms of the electromotive force of a Weston cell at 20 °C (1.0184 Vint), as this type of cell has a lower temperature coefficient than the Clark cell; *several other derived units for use in electrical and magnetic measurements were formally defined: ;International
coulomb The coulomb (symbol: C) is the unit of electric charge in the International System of Units (SI). It is defined to be equal to the electric charge delivered by a 1 ampere current in 1 second, with the elementary charge ''e'' as a defining c ...
:the
electric charge Electric charge (symbol ''q'', sometimes ''Q'') is a physical property of matter that causes it to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative''. Like charges repel each other and ...
transferred by a current of one international ampere in one second;The
coulomb The coulomb (symbol: C) is the unit of electric charge in the International System of Units (SI). It is defined to be equal to the electric charge delivered by a 1 ampere current in 1 second, with the elementary charge ''e'' as a defining c ...
and the
farad The farad (symbol: F) is the unit of electrical capacitance, the ability of a body to store an electrical charge, in the International System of Units, International System of Units (SI), equivalent to 1 coulomb per volt (C/V). It is named afte ...
had been used in earlier B.A. systems of electrical units with slightly different definitions, hence the need to add the qualifier "international".
;International
farad The farad (symbol: F) is the unit of electrical capacitance, the ability of a body to store an electrical charge, in the International System of Units, International System of Units (SI), equivalent to 1 coulomb per volt (C/V). It is named afte ...
:the
capacitance Capacitance is the ability of an object to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized are two closely related ...
of a
capacitor In electrical engineering, a capacitor is a device that stores electrical energy by accumulating electric charges on two closely spaced surfaces that are insulated from each other. The capacitor was originally known as the condenser, a term st ...
charged to a potential of one international volt by one international coulomb of electricity; ;
Joule The joule ( , or ; symbol: J) is the unit of energy in the International System of Units (SI). In terms of SI base units, one joule corresponds to one kilogram- metre squared per second squared One joule is equal to the amount of work d ...
:107 units of
work Work may refer to: * Work (human activity), intentional activity people perform to support themselves, others, or the community ** Manual labour, physical work done by humans ** House work, housework, or homemaking ** Working animal, an ani ...
in the CGS system, represented sufficiently well for practical use by the energy expended in one second by an international ampere in an international ohm; ;
Watt The watt (symbol: W) is the unit of Power (physics), power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantification (science), quantify the rate of Work ...
:107 units of power in the CGS system, represented sufficiently well for practical use by the work done at the rate of one joule per second; ; Henry:the
inductance Inductance is the tendency of an electrical conductor to oppose a change in the electric current flowing through it. The electric current produces a magnetic field around the conductor. The magnetic field strength depends on the magnitude of the ...
in a circuit when an electromotive force induced in this circuit is one international volt, while the inducing current varies at the rate of one ampere per second.


SI units

With advances in the theory of
electromagnetism In physics, electromagnetism is an interaction that occurs between particles with electric charge via electromagnetic fields. The electromagnetic force is one of the four fundamental forces of nature. It is the dominant force in the interacti ...
and in quantity calculus, it became apparent that, in addition to the base units of time, length and mass, a coherent system of units could include only one electromagnetic base unit. The first such system was proposed by Giorgi in 1901:. it used the ohm as the additional base unit in the MKS system, and so is often referred to as the MKSΩ system or the Giorgi system. An additional problem with the CGS system of electrical units, pointed out as early as 1882 by
Oliver Heaviside Oliver Heaviside ( ; 18 May 1850 – 3 February 1925) was an English mathematician and physicist who invented a new technique for solving differential equations (equivalent to the Laplace transform), independently developed vector calculus, an ...
, was that they were not "rationalized", that is they failed to properly take account of
permittivity In electromagnetism, the absolute permittivity, often simply called permittivity and denoted by the Greek letter (epsilon), is a measure of the electric polarizability of a dielectric material. A material with high permittivity polarizes more ...
and permeability as properties of a medium. Giorgi was also a great proponent of rationalization of the electrical units. The choice of electrical unit for the base unit in a rationalized system depends only on practical considerations, particularly the ability to realize the unit accurately and reproducibly. The
ampere The ampere ( , ; symbol: A), often shortened to amp,SI supports only the use of symbols and deprecates the use of abbreviations for units. is the unit of electric current in the International System of Units (SI). One ampere is equal to 1 c ...
rapidly gained support over the ohm, as many national standards laboratories were already realizing the ampere in absolute terms using ampere balances. The
International Electrotechnical Commission The International Electrotechnical Commission (IEC; ) is an international standards organization that prepares and publishes international standards for all electrical, electronics, electronic and related technologies. IEC standards cover a va ...
(IEC) adopted the Giorgi system with the ampere replacing the ohm in 1935, and this choice of base units is often called the MKSA system. The
International Committee for Weights and Measures The General Conference on Weights and Measures (abbreviated CGPM from the ) is the supreme authority of the International Bureau of Weights and Measures (BIPM), the intergovernmental organization established in 1875 under the terms of the Metre C ...
(CIPM) approved a new set of definitions for electrical units, based on the rationalized MKSA system, in 1946, and these were internationally adopted under the
Metre Convention The Metre Convention (), also known as the Treaty of the Metre, is an international treaty that was signed in Paris on 20 May 1875 by representatives of 17 nations: Argentina, Austria-Hungary, Belgium, Brazil, Denmark, France, German Empire, Ge ...
by the 9th  General Conference on Weights and Measures in 1948. Under this system, which would become the
International System of Units The International System of Units, internationally known by the abbreviation SI (from French ), is the modern form of the metric system and the world's most widely used system of measurement. It is the only system of measurement with official s ...
(SI), the ohm is a derived unit.The ohm is the electric resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere, the conductor not being the seat of any electromotive force. The SI definitions of the electrical units are formally equivalent to the 1908 international definitions, and so there should not have been any change in the size of the units. Nevertheless, the international ohm and the international volt were not usually realized in absolute terms but by reference to a standard resistance and a standard electromotive force respectively. The realizations recommended in 1908 are not exactly equivalent to the absolute definitions: recommended conversion factors. are :1 Ωint ≈ 1.000 49 Ω :1 Vint ≈ 1.000 34 V although slightly different factors may apply for individual standards in national measurement laboratories.Conversion factors for the U.S. national standards (NIST) are 1 Ωint = and 1 Vint = . As the international ampere was usually realized by means of an ampere balance rather than electrolytically, 1 Aint = 1 A. The conversion factor for the "electrolytic" ampere (Aelec) can be calculated from modern values of the
atomic weight Relative atomic mass (symbol: ''A''; sometimes abbreviated RAM or r.a.m.), also known by the deprecated synonym atomic weight, is a dimensionless physical quantity defined as the ratio of the average mass of atoms of a chemical element in a giv ...
of
silver Silver is a chemical element; it has Symbol (chemistry), symbol Ag () and atomic number 47. A soft, whitish-gray, lustrous transition metal, it exhibits the highest electrical conductivity, thermal conductivity, and reflectivity of any metal. ...
and the
Faraday constant In physical chemistry, the Faraday constant (symbol , sometimes stylized as ℱ) is a physical constant defined as the quotient of the total electric charge () by the amount () of elementary charge carriers in any given sample of matter: it ...
: :1 Aelec = 1.000 022(2) A


See also

* Conventional electrical units


Notes and references


Notes


References

{{reflist, 2


External links


Sizes.com
* Metrology History of electrical engineering