SI unit of inductance
Thehenry (symbol:H) is the unit of electricalinductance in theInternational System of Units (SI), defined as 1kg⋅m2⋅s−2⋅A−2.[1][2] If a current of 1ampere flowing through acoil producesflux linkage of 1weber, that coil has a self-inductance of 1 henry. The unit is named afterJoseph Henry (1797–1878), the American scientist who discovered electromagnetic induction independently of and at about the same time asMichael Faraday (1791–1867) in England.[3]
Theinductance of an electric circuit is one henry when anelectric current that is changing at oneampere persecond results in anelectromotive force of onevolt across the inductor:
whereV(t) is the resulting voltage across the circuit,I(t) is the current through the circuit, andL is the inductance of the circuit.
The henry is aderived unit based on four of the seven base units of theInternational System of Units:kilogram (kg),metre (m),second (s), andampere (A).Expressed in combinations of SI units, the henry is:[2]
where:H = henry,kg =kilogram,m =metre,s =second,A =ampere,N =newton,C =coulomb,J =joule,T =tesla,Wb =weber,V =volt,F =farad,Ω =ohm, Hz =hertz, rad =radian (dimensionless quantity)
The International System of Units (SI) specifies that the symbol of a unit named for a person is written with an initial capital letter, while the name is not capitalized in sentence text, except when any word in that position would be capitalized, such as at the beginning of a sentence or in material using title case.
The United StatesNational Institute of Standards and Technology recommends users writing in English to use the plural ashenries.[4]
The inductance of a coil depends on its size, the number of turns, and thepermeability of the material within and surrounding the coil.Formulae can be used to calculate the inductance of many common arrangements of conductors, such as parallel wires, or asolenoid. A small air-core coil used for broadcast AM radio tuning might have an inductance of a few tens of microhenries (~10 μH). A large motor winding with many turns around an iron core may have an inductance of hundreds of henries. The physical size of an inductance is also related to its current carrying and voltage withstand ratings.
