henry

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Symbol H. The SI unit of inductance equal to the inductance of a closed circuit in which an e.m.f. of one volt is produced when the electric current in the circuit varies uniformly at a rate of one ampere per second. It is named after the U.S. physicist Joseph Henry (1797–1878).

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henrys (pl.)

[Etymology: J. Henry; USA 1797-1878] electromagnetic inductance, magnetic permeance. Symbol H. The henrys of inductance of a circuit equal the ratio of the generated electromotive force in volts to the rate of change of the current in amperes per second.

H = V·(A·s^-1)^-1 = V·s·A^-1 = Wb·A^-1.

SI, Metric-m.k.s. (= m²·kg·s^-2·A^-2 in base terms). The following are among the coherent derived units:

• H^-1 for magnetic reluctance;
• H·m^-1 for magnetic permeability.

This shows that the electrical and the magnetic are inseparable facets: inductance in an electric circuit is always associated with the generation of a magnetic field.

Metric-c.g.s. See abhenry; stathenry. See also practical unit.

History

The henry was agreed as a unit, but with the name quadrant, by the International Electrical Conference in 1889, and renamed henry in 1893, joining the already established practical units derived from the absolute units of the e.m.u. system. This was the practical henry, = 10⁹ abhenry. Like all the e.m. units, the abhenry was itself defined ultimately in terms of purely mechanical units.

Because the explicit laboratory specifications established for the ampere, ohm, and volt were subsequently found to be slightly discrepant from the intent, so was the henry, as a unit derived from them. The IEC of 1908 covered the discrepancy by adopting the distinct name international henry. Because of experimental vagaries, the value for conversions is normally referred to as the mean international henry, = 1.000 49~ H. There is also the US international henry, = 1.000 495~ H.

With the adoption of the m.k.s.A. system in 1948, and its basing of electrical units on an ampere compatible with the original absolute units, the modern henry is essentially the old practical henry. Sometimes called the absolute henry, this became identically the henry of the SI.

A unit of measurement of inductance, which is the strength of a magnetic field in an inductor. The symbol for henry is the letter H. See inductance and inductor.

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To convert from henries to:

millihenries, multiply by 1000.

Convert:  Into:  millihenries

Result:

The basic unit of inductance.

(pl. henrys or henries)

symbol: H; the SI derived unit of inductance, equal to the inductance of a closed circuit with a magnetic flux of 1 weber per ampere of current,
H = Wb A⁻¹ s = m² kg s⁻² A⁻²
[After Joseph Henry (1797 — 1878), US physicist.]

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For a list of words related to henry, see:

• Electricity and Magnetism - henry: unit of inductance in closed circuit where electromotive force of one volt is produced by current variation of one ampere per second
• Electricity and Electronics - henry: unit of inductance in which a variation of one ampere per second produces an induced electromotive force of one volt

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