henry
Inductanceexact by definition
The henry is the SI unit of inductance, defined as the inductance of a circuit in which a current changing at one ampere per second induces one volt of electromotive force. As a coherent derived unit it equals one weber per ampere, or one volt second per ampere, exactly. Like the farad it is a large unit, so most practical components are rated in microhenries or millihenries.
| 1 H | 1,000 mH |
Named for Joseph Henry, who discovered self-inductance in the late 1820s independently of Faraday's work on mutual induction and built the electromagnets that made the telegraph practical.
About the henry
An inductor resists change in current the way a mass resists change in velocity, and the defining relation says exactly that: \(V = L \, di/dt\). Push current through a coil and the growing magnetic field induces a back-EMF opposing the change; try to stop the current abruptly and the field collapses into a voltage spike that can be hundreds of times the supply. That spike is why a flyback diode sits across every relay coil and solenoid in well-designed equipment, and why switching off an inductive load draws an arc at the contacts.
Energy is stored in the field as \(E = \ frac{1}{2}LI^2\), the mirror image of the capacitor's \(\ frac{1}{2}CV^2\). Put the two together and you get an oscillator: an LC pair rings at \(f = 1/(2\pi\sqrt{LC})\), the basis of every radio tuner, filter and clock recovery circuit. Inductance grows with the square of the turn count and with the permeability of the core, which is why a ferrite or iron core buys orders of magnitude over air, and why a core that saturates makes the inductance collapse just when the current is highest.