Since the redefinition of SI units in 2019 (when the values of e and h were fixed as defined quantities), μ0 is an experimentally determined constant, its value being proportional to the dimensionless fine-structure constant, which is known to a relative uncertainty of about 1.5×10−10, with no other dependencies with experimental uncertainty.
Its value in SI units as recommended by CODATA 2018 (published in May 2019) is:
μ0 = 1.25663706212(19)×10−6 N⋅A−2
From 1948 to 2019, μ0 had a defined value (per the former definition of the SI ampere), equal to:
μ0 = 4π×10−7 H/m = 1.25663706143...×10−6N/A2 (1 henry per metre = 1 newton per square ampere = 1 tesla metre per ampere)
The deviation of the recommended measured value from the former defined value is statistically significant, at about 3.6σ, listed as μ0/(4π×10−7 N⋅A−2) − 1 = (5.5±1.5)×10−10.
Two thin, straight, stationary, parallel wires, a distance r apart in free space, each carrying a currentI, will exert a force on each other. Ampère's force law states that the magnetic force Fm per length L is given by
From 1948 until 2019 the ampere was defined as "that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2×10−7 newton per meter of length". This is equivalent to a definition of of exactly 4π×10−7H/m.[a], since
The current in this definition needed to be measured with a known weight and known separation of the wires, defined in terms of the international standards of mass, length and time in order to produce a standard for the ampere (and this is what the Kibble balance was designed for). In the 2019 redefinition of the SI base units, the ampere is defined exactly in terms of the elementary charge and the second, and the value of is determined experimentally; 4π × 1.00000000055(15)×10−7 H⋅m−1 is a recently measured value in the new system (and the Kibble balance has become an instrument for measuring weight from a known current, rather than measuring current from a known weight).
Standards organizations have recently moved to magnetic constant as the preferred name for μ0, although the older name continues to be listed as a synonym. Historically, the constant μ0 has had different names. In the 1987 IUPAP Red book, for example, this constant was still called permeability of vacuum. Another, now rather rare and obsolete, term is "magnetic permittivity of vacuum". See, for example, Servant et al. The term "vacuum permeability" (and variations thereof, such as "permeability of free space") remains very widespread.
The name "magnetic constant" was used by standards organizations in order to avoid use of the terms "permeability" and "vacuum", which have physical meanings. This change of preferred name had been made because μ0 was a defined value, and was not the result of experimental measurement (see below). In the new SI system, the permeability of vacuum no longer has a defined value, but is a measured quantity, with an uncertainty related to that of the (measured) dimensionless fine structure constant.
Systems of units and historical origin of value of μ0Edit
In principle, there are several equation systems that could be used to set up a system of electrical quantities and units.
Since the late 19th century, the fundamental definitions of current units have been related to the definitions of mass, length, and time units, using Ampère's force law. However, the precise way in which this has "officially" been done has changed many times, as measurement techniques and thinking on the topic developed.
The overall history of the unit of electric current, and of the related question of how to define a set of equations for describing electromagnetic phenomena, is very complicated. Briefly, the basic reason why μ0 has the value it does is as follows.
Ampère's force law describes the experimentally-derived fact that, for two thin, straight, stationary, parallel wires, a distance r apart, in each of which a current I flows, the force per unit length, Fm/L, that one wire exerts upon the other in the vacuum of free space would be given by
Writing the constant of proportionality as km gives
The form of km needs to be chosen in order to set up a system of equations, and a value then needs to be allocated in order to define the unit of current.
In the old "electromagnetic (emu)" system of equations defined in the late 19th century, km was chosen to be a pure number, 2, distance was measured in centimetres, force was measured in the cgs unit dyne, and the currents defined by this equation were measured in the "electromagnetic unit (emu) of current" (also called the "abampere"). A practical unit to be used by electricians and engineers, the ampere, was then defined as equal to one tenth of the electromagnetic unit of current.
In another system, the "rationalized metre–kilogram–second (rmks) system" (or alternatively the "metre–kilogram–second–ampere (mksa) system"), km is written as μ0/2π, where μ0 is a measurement-system constant called the "magnetic constant".[b]
The value of μ0 was chosen such that the rmks unit of current is equal in size to the ampere in the emu system: μ0 was defined to be 4π × 10−7H/m.[a]
Historically, several different systems (including the two described above) were in use simultaneously. In particular, physicists and engineers used different systems, and physicists used three different systems for different parts of physics theory and a fourth different system (the engineers' system) for laboratory experiments. In 1948, international decisions were made by standards organizations to adopt the rmks system, and its related set of electrical quantities and units, as the single main international system for describing electromagnetic phenomena in the International System of Units.
Ampère's law as stated above describes a physical property of the world. However, the choices about the form of km and the value of μ0 are totally human decisions, taken by international bodies composed of representatives of the national standards organizations of all participating countries. The parameter μ0 is a measurement-system constant, not a physical constant that can be measured. It does not, in any meaningful sense, describe a physical property of the vacuum.[c] This is why the relevant Standards Organizations prefer the name "magnetic constant", rather than any name that carries the hidden and misleading implication that μ0 describes some physical property.
This relation can be derived using Maxwell's equations of classical electromagnetism in the medium of classical vacuum, but this relation is used by BIPM (International Bureau of Weights and Measures) and NIST (National Institute of Standards and Technology) as a definition of ε0 in terms of the defined numerical values for c and μ0, and is not presented as a derived result contingent upon the validity of Maxwell's equations.
^ abThis choice defines the SI unit of current, the ampere: "Unit of electric current (ampere)". Historical context of the SI. NIST. Retrieved 2007-08-11.
^The decision to explicitly include the factor of 2π in km stems from the "rationalization" of the equations used to describe physical electromagnetic phenomena.
^The magnetic permeability of a realizable vacuum (such as outer space, or ultra-high vacuum), which is measurable at least in principle, is distinct from the defined parameter μ0.
^"Convocationde la Conférence générale des poids et mesures (26e réunion)" (PDF).
^Parker, Richard H.; Yu, Chenghui; Zhong, Weicheng; Estey, Brian; Müller, Holger (2018-04-13). "Measurement of the fine-structure constant as a test of the Standard Model". Science. 360 (6385): 191–195. arXiv:1812.04130. Bibcode:2018Sci...360..191P. doi:10.1126/science.aap7706. ISSN 0036-8075. PMID29650669. S2CID 4875011.
^Davis, Richard S. (2017). "Determining the value of the fine-structure constant from a current balance: Getting acquainted with some upcoming changes to the SI". American Journal of Physics. 85 (5): 364–368. arXiv:1610.02910. Bibcode:2017AmJPh..85..364D. doi:10.1119/1.4976701. ISSN 0002-9505. S2CID 119283799.
^Rosen, Joe (2004). "Permeability (Physics)". Encyclopedia of Physics. Facts on File science library. New York: Facts On File. ISBN 9780816049745. Retrieved 2010-02-04.(registration required)
^Magnetic Permeability, and Analogues in Electro-static Induction, Conduction of Heat, and Fluid Motion, March 1872.
^See for example equation 25-14 in Tipler, Paul A. (1992). Physics for Scientists and Engineers, Third Edition, Extended Version. New York, NY: Worth Publishers. p. 826. ISBN 978-0-87901-434-6.
^See Table 1 in Mohr, Peter J; Taylor, Barry N; Newell, David B (2008). "CODATA Recommended Values of the Fundamental Physical Constants: 2006" (PDF). Reviews of Modern Physics. 80 (2): 633–730. arXiv:0801.0028. Bibcode:2008RvMP...80..633M. CiteSeerX10.1.1.150.1225. doi:10.1103/RevModPhys.80.633.
^SUNAMCO (1987). "Recommended values of the fundamental physical constants" (PDF). Symbols, Units, Nomenclature and Fundamental Constants in Physics. p. 54.
^Lalanne, J.-R.; Carmona, F.; Servant, L. (1999). Optical spectroscopies of electronic absorption. World Scientific Series in Contemporary Chemical Physics. Vol. 17. p. 10. Bibcode:1999WSSCP..17.....L. doi:10.1142/4088. ISBN 978-981-02-3861-2.
^For an introduction to the subject of choices for independent units, see John David Jackson (1998). Classical electrodynamics (Third ed.). New York: Wiley. p. 154. ISBN 978-0-471-30932-1.
^"2018 CODATA Value: speed of light in vacuum". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved 2019-05-20.
^The exact numerical value is found at:
"Electric constant, ε0". NIST reference on constants, units, and uncertainty: Fundamental physical constants. NIST. Retrieved 2012-01-22. This formula determining the exact value of ε0 is found in Table 1, p. 637 of Mohr, Peter J; Taylor, Barry N; Newell, David B (2008). "CODATA recommended values of the fundamental physical constants: 2006" (PDF). Reviews of Modern Physics. 80 (2): 633–730. arXiv:0801.0028. Bibcode:2008RvMP...80..633M. CiteSeerX10.1.1.150.1225. doi:10.1103/RevModPhys.80.633.