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 vacuum permittivity ： ウィキペディア英語版
vacuum permittivity

The physical constant , commonly called the vacuum permittivity, permittivity of free space or electric constant, is an ideal, (baseline) physical constant, which is the value of the absolute dielectric permittivity of classical vacuum. Its value is
0 = 8.854 187 817... × 10−12 F/m (farads per metre).
It is the capability of the vacuum to permit electric field lines.
This constant relates the units for electric charge to mechanical quantities such as length and force.〔
.〕 For example, the force between two separated electric charges (in the vacuum of classical electromagnetism) is given by Coulomb's law:
:$\ F_C = \frac \frac$
where ''q''1 and ''q''2 are the charges, and ''r'' is the distance between them. Likewise, ''ε''0 appears in Maxwell's equations, which describe the properties of electric and magnetic fields and electromagnetic radiation, and relate them to their sources.
==Value==
The value of ''ε''0 is currently ''defined'' by the formula〔
The exact numerical value is found at:
This formula determining the exact value of ''ε''0 is found in Table 1, p. 637 of

:$\varepsilon_0 =\frac$
where ''c'' is the defined value for the speed of light in classical vacuum in SI units,〔
Quote from NIST: "The symbol ''c'' is the conventional symbol for the speed of light in vacuum. " See (NIST ''Special Publication 330'', p. 18 )
〕 and ''μ''0 is the parameter that international Standards Organizations call the "magnetic constant" (commonly called vacuum permeability). Since ''μ''0 has the ''defined'' value 4π × 10−7 Hm−1,〔See the last sentence of the (NIST definition of ampere ).〕 and ''c'' has the ''defined'' value  m⋅s−1,〔See the last sentence of the (NIST definition of meter ).〕 it follows that ''ε''0 has a ''defined'' value given approximately by
:''ε''0 ≈ ... × 10−12 Fm−1 (or A2s4kg−1m−3 in SI base units, or C2N−1m−2 or CV−1m−1 using other SI coherent units).〔.〕〔A summary of the definitions of ''c'', ''μ''0 and ''ε''0 is provided in the 2006 CODATA Report: (CODATA report, pp. 6–7 )〕
The historical origins of the electric constant ''ε''0, and its value, are explained in more detail below.

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