ampereturn

An ampereturn, frequently called an ampere-turn (symbol AT), is the unit of magnetomotive force (MMF) in electromagnetism. It is defined as the product of the current in amperes (I) and the number of turns (N) in a coil surrounding a magnetic circuit: MMF = N × I. The SI unit is the ampere-turn, and the plural form is ampere-turns; the term is commonly used in both engineering and physics contexts.

In magnetic circuit theory, the MMF is the line integral of the magnetic field H around a closed path: MMF = ∮ H · dl. For a simple coil wound around a core with a relatively uniform H, MMF ≈ H × l, where l is the core length. Magnetic flux Φ relates to MMF through the core’s reluctance Rm via Φ = MMF / Rm. Thus ampereturns provide a scalar measure of the driving force that produces magnetic flux in devices and circuits, independent of geometric details.

Ampereturns are widely used in the design of transformers, inductors, electromagnets, and other magnetic devices. For example, a coil with 250 turns carrying 1.5 A has an MMF of 375 ampere-turns. In practice, the effective MMF depends on material properties such as permeability and saturation, and on fringing and leakage in non-ideal geometries, so real-world calculations often use magnetic circuit models and empirical data.

The term honors André-Marie Ampère, a founder of electromagnetism. While related to current, an ampereturn combines current and turns to reflect the magnetomotive driving force; it is not a standalone measure of current.