Overview

The henry (symbol H) is the International System of Units (SI) measure of electrical inductance. Inductance quantifies the tendency of an electrical conductor or circuit to oppose changes in current by producing an electromotive force. The unit honours American scientist Joseph Henry and appears in contexts ranging from basic physics to practical electronics.

Definition and relations

One henry is defined so that a change in current of one ampere per second through an inductor induces one volt of electromotive force across it. Equivalently, 1 H = 1 V·s/A = 1 Wb/A. In terms of SI base units, 1 H = kg·m2·s−2·A−2. These relations make the henry useful for converting between electrical, magnetic and mechanical quantities.

Physical meaning and formulas

Inductance reflects the magnetic coupling of a circuit: a larger inductance produces a larger induced voltage for a given rate of change of current. The energy stored in an inductor with current I is E = 1/2·L·I2. In simple series circuits, the time constant for current change is τ = L/R, where R is resistance. Inductance therefore governs transient responses, filtering behaviour, and resonant frequencies when combined with capacitance.

History and naming

The unit commemorates Joseph Henry (1797–1878), who discovered electromagnetic induction; Michael Faraday (1791–1867) made independent, nearly simultaneous discoveries in England. The name "henry" is written in lower case as are other unit names, while the symbol uses an upper-case H. Guidance from the National Institute of Standards and Technology can be consulted for usage and plural form ("henries") via NIST.

Uses and examples

  • Electronic components: inductors for filtering, tuning, and energy storage in power supplies and radio-frequency circuits.
  • Transformers and motors: windings and magnetic cores characterized by inductance determine coupling and performance.
  • Measurement and industry: magnetic sensors, MRI coils, and large-scale power-system inductors use henry-scale values.

Notable facts and conventions

The magnetic constant (vacuum permeability), often denoted μ0, is commonly cited as 4π×10−7 H/m; see entries on magnetic permeability and vacuum for background. The phenomenon that gives rise to inductance is called electromagnetic induction, historically associated with Faraday as well as Henry. The henry appears alongside derived units such as the weber (magnetic flux) and the volt, forming a consistent framework for describing electromagnetic devices and their behaviour.