# Henry (unit)

Henry
An inductor composed of a wire wound around a magnetic core used to confine and guide the induced magnetic field.
Unit information
Unit system SI derived unit
Unit of Inductance
Symbol H
Named after Joseph Henry
In SI base units: kgm2s-2A-2

The henry (symbol H) is the unit of electrical inductance in the International System of Units.[1] The unit 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.[2] The magnetic permeability of vacuum is 4π×10−7 H m-1 (henry per metre).

The United States National Institute of Standards and Technology recommends English-speaking users of SI to write the plural as henries.[3]:31

## Definition

The inductance of an electric circuit is one henry when an electric current that is changing at one ampere per second results in an electromotive force of one volt across the inductor :

$\displaystyle v(t)= L \frac{di}{dt}$,

where v(t) denotes the resulting voltage across the circuit, i(t) is the current through the circuit, and L is the inductance of the circuit.

The henry is a derived unit based on four of the seven base units of the International System of Units: kilogram (kg), meter (m), second (s), and ampere (A). Expressed in combinations of SI units, the henry is:[4]

$\mbox{H} = \dfrac{\mbox{kg} \cdot \mbox{m}^2}{\mbox{s}^{2} \cdot \mbox{A}^2} = \dfrac{\mbox{kg} \cdot \mbox{m}^2}{\mbox{C}^2} = \dfrac{\mbox{J}}{\mbox{A}^2} = \dfrac{\mbox{T} \cdot \mbox{m}^2}{\mbox{A}} = \dfrac{\mbox{Wb}}{\mbox{A}} = \dfrac{\mbox{V} \cdot \mbox{s}}{\mbox{A}} = \dfrac{\mbox{s}^2}{\mbox{F}} = \dfrac{\mbox{1}}{\mbox{F} \cdot \mbox{Hz}^2} = \Omega \cdot \mbox{s}$

in which the following additional derived units occur: coulomb (C), farad (F), joule (J), weber (Wb), tesla (T), volt (V), hertz (Hz), and ohm (Ω).

## Yrneh

The term yrneh, obtained by reversing the letters of henry, is said to have been used as a measure of reluctance, the reciprocal of the inductance. It is defined as: 1 yrneh = 1 henry-1.[5] This word is not listed in the Oxford English Dictionary,[6] however it is listed in Chambers Dictionary (11th edition).