Henry

The henry (symbol H) is the SI unit of inductance, both self- and mutual. Its defining relation is V = L dI/dt: a one-henry coil develops a one-volt back-EMF when its current changes at one ampere per second. Equivalently, 1 H = 1 Wb/A (one weber of self-flux per ampere of current) = 1 V·s/A = 1 Ω·s.

Typical inductances span many orders of magnitude. Small RF air-core coils in AM-radio tuners are on the order of microhenries (µH). Power-frequency transformer primaries typically have inductances of tens of millihenries (mH) to henries, depending on core geometry. Industrial filter chokes used to smooth rectified power-supply ripple can reach tens of henries in a single unit. At the other extreme, superconducting storage inductors proposed for grid-scale energy storage would have inductances in the kilohenry range, storing energy at the mass-of-a-car scale. The henry is a convenient size for ordinary electrical engineering.

The unit was named for Joseph Henry, the American physicist who independently discovered self-induction in the early 1830s. In the original 1881 SI-precursor agreements, the unit of inductance was called the "quad" or simply left unnamed; Joseph Henry's name was attached by the International Electrotechnical Commission in 1893, a decade and a half after his death, partly as a belated acknowledgement that Faraday-priority alone did not capture the historical reality. The henry has remained the SI unit of inductance unchanged ever since.