Identify the proper unit of magnetic reluctance for a magnetic circuit.

Introduction / Context:
Reluctance is the magnetic analogue of electrical resistance. It quantifies how strongly a magnetic circuit opposes the establishment of magnetic flux for a given magnetomotive force (MMF). Understanding its unit clarifies relationships in magnetic circuits.

Given Data / Assumptions:

• Reluctance symbol: script R (often Rm or ℜ).
• Definition: ℜ = MMF / phi.
• MMF in ampere–turns (At), flux phi in webers (Wb).

Concept / Approach:

From the definition ℜ = MMF / phi, the unit must be (ampere–turns) per weber, i.e., At/Wb. Other magnetic quantities have different SI units (e.g., tesla for flux density, Wb for flux, H/m for permeability).

Step-by-Step Solution:

Verification / Alternative check:

Also, using ℜ = l / (µ * A) leads to units: m / (H/m * m^2) = m / (H * m) = 1/H. Since 1 H = Wb/A, 1/H = A/Wb. Considering turns as unitless count, this is consistent with At/Wb in practical usage.

Why Other Options Are Wrong:

Tesla is flux density, At/m is field strength unit (H), and Wb is flux. H/m is permeability, not reluctance.

Common Pitfalls:

Confusing reluctance with reluctivity or permeability, and mixing up the roles of H, B, and phi in magnetic circuits.

Final Answer:

At/Wb