Henry (unit)

An inductor.

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).

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.

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

units
A = amp
V = volt
C = coulomb
J = Joule
Wb = weber
kg = kilogram
m = meter
s = second

SI multiples

SI multiples of henry (H)

Submultiples			Multiples
Value	SI symbol	Name	Value	SI 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
10−27 H	rH	rontohenry	1027 H	RH	ronnahenry
10−30 H	qH	quectohenry	1030 H	QH	quettahenry

The henry is named after Joseph Henry. As with every SI unit named for a person, its symbol starts with an upper case letter (H), but when written in full, it follows the rules for capitalisation of a common noun; i.e., henry becomes capitalised at the beginning of a sentence and in titles but is otherwise in lower case.

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

See also

• Impedance
• Inductor
• Wb