Here is a surprising fact: a bare magnet on its own is a weak device. Put the same magnet inside a steel housing, and it pulls several times harder. The magnet did not change — the magnetic circuit around it did. This article explains the two ideas behind that trick: permeability and reluctance.
Permeability: how easily flux flows
Think of magnetic flux — the flow of magnetism — as water. Permeability is how easily a material lets that flow pass:
- Air: poor — its permeability is 1, the lowest there is.
- Soft steel: a flux superhighway, hundreds to thousands of times better than air.
- Sintered NdFeB: barely better than air — a magnet makes flux, but it does not conduct it well.
Reluctance: magnetic resistance
Reluctance is what slows flux down — the magnetic version of electrical resistance. Long paths, narrow cross-sections and air gaps all add reluctance. And flux always takes the easiest path available.
The air gap problem
Air has the lowest permeability of all, so an air gap is pure resistance. A gap of just 1 mm can resist as much as a metre of steel. That is why holding force collapses the moment a magnet lifts off the steel surface — the tiny gap adds enormous reluctance, and most of the flux stops doing useful work.
It also explains why magnets come with steel return paths — pot magnets, motor housings, steel brackets: the steel gives flux an easy road home, so far more of it does useful work.
Practical rules
- Keep mating surfaces flat and fully covered — every millimetre of gap you remove buys more pull than any grade upgrade.
- Put steel where you want flux to go; keep non-magnetic stainless steel (like 304) out of the flux path.
- Watch for small steel bridges that can steal flux from the working gap.
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