Henry

The henry or henry (symbol: H) is the unit for electrical inductance in the International System of Units.

Definition

It is the electrical inductance of a closed circuit in which an electromotive force of 1 volt is produced, when the electrical current that runs through the circuit varies uniformly at the rate of one ampere per second. Its name was given in honor of the American physicist Joseph Henry.

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 units of the International System, the henry is:

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

It is used to measure the self-induction coefficient (L) in the coils of electrical circuits, and the mutual induction coefficient (M) between two electromagnetically coupled coils.

Uses

The inductance of a coil depends on its size, the number of turns, and the permeability of the material in and around the coil. Formulas can be used to calculate the inductance of many common conductor arrangements, such as parallel cables or a solenoid. A small air-core coil used for AM radio tuning may have an inductance of a few tens of microhenries. A large motor winding with many turns around an iron core can have an inductance of hundreds of henries. The physical size of an inductor is also related to its current carrying and voltage withstand rating.