# Henry (unit): Wikis

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# Encyclopedia

### From Wikipedia, the free encyclopedia

In physics, and electronics, the henry (symbol: H) is the SI unit of inductance. Originally the symbol was actually an upside down T but due to the fact no such symbol existed in standard typesets, the symbol was changed to a H. 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).

National Institute of Standards and Technology provides guidance for American users of SI to write the plural as henries.

## 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.

$\mbox{H} = \dfrac{\mbox{m}^2 \cdot \mbox{kg}}{\mbox{s}^{2} \cdot \mbox{A}^2} = \dfrac{\mbox{Wb}}{\mbox{A}} = \dfrac{\mbox{V} \cdot \mbox{s}}{\mbox{A}} = \dfrac{\mbox{J/C} \cdot \mbox{s}}{\mbox{C/s}} = \dfrac{\mbox{J} \cdot \mbox{s}^2}{\mbox{C}^2} = \dfrac{\mbox{m}^2 \cdot \mbox{kg}}{\mbox{C}^2} = \Omega \cdot s$

units
A = ampere
V = volt
C = coulomb
J = joule
Wb = weber
kg = kilogram
m = meter
s = second
Ω = ohm

## See also

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