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# Henry (unit)

The henry (symbol: H) is the SI unit of inductance. It is named after Joseph Henry (1797-1878), the American scientist who discovered electromagnetic induction independently of and at about the same time as Michael Faraday (1791-1867) in England. The magnetic permeability of the vacuum is 4π×10−7 H/m (henry per metre).

## Definition

If the rate of change of current in a circuit is one ampere per second and the resulting electromotive force is one volt, then the inductance of the circuit is one henry. $H = \dfrac{\mbox{m}^2 \cdot \mbox{kg}}{\mbox{s}^{2} \cdot \mbox{A}^2} = \dfrac{\mbox{Wb}}{\mbox{A}} = \dfrac{\mbox{V} \cdot \mbox{s}}{\mbox{A}} = \dfrac{\mbox{m}^2 \cdot \mbox{kg}}{\mbox{C}^2}$

## SI multiples

SI multiples for henry (H)
Submultiples Multiples
Value Symbol Name Value Symbol Name
10–1 H dH decihenry 101 H daH decahenry
10–2 H cH centihenry 102 H hH hectohenry
10–3 H mH millihenry 103 H kH kilohenry
10–6 H µH microhenry 106 H MH megahenry
10–9 H nH nanohenry 109 H GH gigahenry
10–12 H pH picohenry 1012 H TH terahenry
10–15 H fH femtohenry 1015 H PH petahenry
10–18 H aH attohenry 1018 H EH exahenry
10–21 H zH zeptohenry 1021 H ZH zettahenry
10–24 H yH yoctohenry 1024 H YH yottahenry

 This SI unit is named after Joseph Henry. As with all SI units whose names are derived from the proper name of a person, the first letter of its symbol is uppercase (H). But when an SI unit is spelled out, it should always be written in lowercase (henry), unless it begins a sentence or is the name "degree Celsius". — Based on The International System of Units, section 5.2.