Surface field depends on where you hold the probe. Magnetic moment does not — it describes how strong a magnet is as a whole source of field. That is why factories use it to compare magnets and check production batches.
What magnetic moment is
Think of magnetic moment as the magnet's engine size: one number for how strong it is as a source of field, independent of probe position or shape. It is measured with the draw-through method — a fluxmeter connected to a Helmholtz coil pair, with the magnet drawn through the coil. The coil integrates the whole magnet in one shot, so the result ignores surface condition, local defects, and who holds the probe.
Moment vs. flux
Flux is the total field through a surface; moment is the source strength that produces it. On a calibrated coil the two are proportional, so they are equally reliable. Both beat surface gauss, which changes with shape, size, probe and gaussmeter.
Why moment is the fair comparator
- Same material, different shapes? Surface readings differ; moment does not.
- Two operators, two gaussmeters? One calibrated coil gives comparable moments; surface readings rarely do.
- Big wind and EV parts? Draw-through moment is the standard acceptance check.
| Method | Measures | Depends on | Typical use |
|---|---|---|---|
| Gaussmeter (Hall probe) | Surface field | Shape, position, probe | Quick in-line checks |
| Fluxmeter + Helmholtz coil | Flux | Magnet only | Batch QC |
| Draw-through, calibrated coil | Magnetic moment | Magnet only | Factory acceptance |
Moment measurement follows IEC 60404-14; material grades follow closed-circuit tests.
What to specify
For materials, use grade parameters (Br, Hcj, (BH)max). For finished parts, agree a moment or flux limit on a calibrated coil — not surface gauss.
Read on: surface field vs. remanence vs. flux → · the four key parameters →