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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.

[edit] 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

[edit] SI multiples

SI multiples for henry (H)
Value	Symbol	Name	Value	Symbol	Name
Submultiples			Multiples
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 every SI unit whose name is derived from the proper name of a person, the first letter of its symbol is uppercase (H). When an SI unit is spelled out in English, it should always begin with a lowercase letter (henry), except where any word would be capitalized, such as at the beginning of a sentence or in capitalized material such as a title. Note that "degree Celsius" conforms to this rule because the "d" is lowercase.

—Based on The International System of Units, section 5.2.

Since the Greek letter μ caused printing difficulties, the notation uH can be found as a substitute for μH in electronics literature.

[edit] See also