farad

n. (Abbr. F)
The unit of capacitance in the meter-kilogram-second system equal to the capacitance of a capacitor having an equal and opposite charge of 1 coulomb on each plate and a potential difference of 1 volt between the plates.

[After Michael FARADAY.]

Symbol F. The SI unit of capacitance, being the capacitance of a capacitor that, if charged with one coulomb, has a potential difference of one volt between its plates. 1 F = 1 C V⁻¹. The farad itself is too large for most applications; the practical unit is the microfarad (10⁻⁶ F). The unit is named after Michael Faraday.

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[Etymology: M. Faraday; UK 1791-1867] electric capacitance Symbol F. The farads of a capacitor equal the ratio of charge in coulombs to the rise in the potential across its plates in volts, identically the number of ampere·seconds per volt of rise in the potential across the plates of the capacitor. For a steady current = C·V^-1, and identically s·A·V^-1.

SI, Metric-m.k.s.A. 1948 (= m^-2·kg^-1·s⁴·A² in base terms). The following is among the coherent derived units:

• F·m^-1 for dielectric permittivity.

The capacitance of a typical piece of equipment is less than 1 μF, resulting in the nF and the pF being common, the latter sometimes called vernacularly a ‘puff’. Because of MF (now correctly the symbol for the massive megafarad) and mf having at some time been common symbols for microfarad, the millifarad (correctly mF) is rarely referred to; quantities that would merit the unit are expressed in microfarads, e.g. 1 200 μF rather than 12 mF.

Metric-c.g.s. See abfarad; statfarad. See also practical unit.

History

Sometimes distinguished as the Latimer-Clark farad, such a unit had been in use in the UK since 1867 at the size of the current μF, being about the capacitance of 2 000 ft of submarine cable of the day. International agreement came, along with that for related units and the use of the c.g.s.-system, in 1881 at the first International Electrical Conference,
[Nature Vol. 24, 512 (1881)] as the ‘the capacity defined by the condition that a coulomb in a farad gives a volt’, with the implication that there be both an absolute form and a corresponding practical unit. The absolute form, later discriminated as the abfarad, falls within the e.m.u. system, and is fundamentally definable in terms of purely mechanical units. The practical farad = 10^-9 abfarad.

The creation of explicit laboratory definitions of the ampere, ohm, and volt, which were subsequently found to be slightly discrepant from those intended, gives a slightly altered farad as a unit derived from them. The IEC of 1908 covered the discrepancy by adopting the distinct name international farad. Because of experimental vagaries, the value for conversions is normally referred to as the mean international farad; = 0.999 510~ F. There is also the US international farad, = 0.999 505~ F, defined by Congress in an Act of 1894.

With the implementation of the Metric-m.k.s.A. system in 1948, and its basing of electrical units on an ampere compatible with the original absolute units, the modern farad became essentially the old practical farad. Sometimes called the absolute farad, it became identically the farad of the SI.

1946	CIPM ‘Farad (unit of capacitance) The farad is the capacitance of a capacitor between the plates of which there appears a potential difference of 1 volt when it is charged by a quantity of electricity of 1 coulomb.’see note below

[Le Système International d'Unités (Sèvres, France: Bureau International de Poids et Mesures, 1985)]

To convert from farads to:

microfarads, multiply by 1000000.

Convert:  Into:  microfarads

Result:

The basic unit of capacitance.

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