Henry (unit)

The henry (symbol: H) is the SI derived unit of electrical inductance.[1] If a current of 1 ampere flowing through the coil produces flux linkage of 1 weber turn, the coil has a self inductance of 1 henry.‌ 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]

An inductor composed of a wire wound around a magnetic core used to confine and guide the induced magnetic field.
General information
Unit systemSI derived unit
Unit ofInductance
Named afterJoseph Henry
In SI base units:kgm2s−2A−2


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:


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), metre (m), second (s), and ampere (A). Expressed in combinations of SI units, the henry is:[3]

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).


The International System of Units (SI) specifies to write the symbol of a unit named for a person 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 States National Institute of Standards and Technology recommends users writing in English to use the plural as henries.[4]:31


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

See also


  1. ^ Rowlett, Russ. "How Many? A Dictionary of Units of Measurement". University of North Carolina at Chapel Hill.
  2. ^ Herbert S. Bailey, Jr. "A Princeton Companion".
  3. ^ "Essentials of the SI: Base & derived units". The NIST Reference on Constants, Units and Uncertainty. National Institute of Standards and Technology.
  4. ^ Ambler Thompson & Barry N. Taylor (2008). "NIST Special Publication 811: Guide for the Use of the International System of Units (SI)" (PDF). National Institute of Standards and Technology. Retrieved 2013-03-21.
Glossary of electrical and electronics engineering

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Henry's may refer to:

Henry's Amazing Animals - Disney Channel children's program

Henry's Amazing Golden Gecko Awards

Henry's Anger - Canberra heavy metal band

Henry's Fork Caldera - Caldera in Yellowstone National Park

Henry's Cat - animated children's television show

Henry's (clothiers) - Wichita company and AAU powerhouse

Henry's Dream - Nick Cave and the Bad Seeds album

Henry's Dress - Rock band from New Mexico

Henry's (electronics retailer) - Canadian retailer

Henry's Farmers Market (formerly Boney's Market) - Southern California retailer

Henry's House - London public relations firm

Henry's Knob - Superfund site in South Carolina

Henry's Lake National Forest - National forest in Idaho

Henry's law - a chemistry gas law

Henry's Grove - Berlin, Worcester County, Maryland

Sir Henry's - nightclub in Cork, Ireland

St. Henry's Church - church in Bayonne, New Jersey

Henry's Hamburgers - an american restaurant chain

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Joseph Henry

Joseph Henry (December 17, 1797 – May 13, 1878) was an American scientist who served as the first Secretary of the Smithsonian Institution. He was the secretary for the National Institute for the Promotion of Science, a precursor of the Smithsonian Institution. He was highly regarded during his lifetime. While building electromagnets, Henry discovered the electromagnetic phenomenon of self-inductance. He also discovered mutual inductance independently of Michael Faraday, though Faraday was the first to make the discovery and publish his results. Henry developed the electromagnet into a practical device. He invented a precursor to the electric doorbell (specifically a bell that could be rung at a distance via an electric wire, 1831) and electric relay (1835). The SI unit of inductance, the Henry, is named in his honor. Henry's work on the electromagnetic relay was the basis of the practical electrical telegraph, invented by Samuel F. B. Morse and Sir Charles Wheatstone, separately.

Base units
Derived units
with special names
Other accepted units
See also

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