Magnet field and temperature calculator
What a gaussmeter reads above a magnet, and what happens to that reading as the magnet gets hot. The load line and the demagnetization curve move together, and when they cross below the knee, part of the loss never comes back.
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The magnet
Pick a shape and type its size. Everything else on the page follows from this.
- On the pole face
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- Pull force on steel
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- Permeance coefficient
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Demagnetization curve
Field against temperature
What this is doing
The load line
A magnet sitting on its own in air is working against its own stray field. The geometry sets how hard: the permeance coefficient Pc = (1−N)/N, where N is the volume-averaged demagnetizing factor, computed here from the shape rather than read off a chart. A cube comes out at exactly Pc = 2. Thin and wide means a low Pc and an operating point far down the curve; long and thin means a high one.
The knee
Heat a magnet and its remanence drifts down a little while its coercivity falls a lot. The curve pulls in toward the origin, the load line stays where it is, and at some temperature the crossing slips past the knee. Above that point the magnet does not return to where it started when it cools — it recoils along a lower line, and the gap is the irreversible loss. Push the slider up and back down to watch it happen.
The field at a distance
Closed-form on-axis solutions for a uniformly polarized disc, ring and block. The polarization used is the one at the operating point, not the catalog remanence, so these numbers sit a percent or two below the calculators that assume Br everywhere. On a ring the reading is taken on the axis, where the flux returns through the bore — close to the face it points backwards.
What it is not
A uniform-magnetization model, so it says nothing about corners, edge softening or a partly magnetized part. Pull force assumes flush contact with no air gap, paint or surface finish, against steel that saturates at 1.2 T — both at the interface and, usually the binding one, in the plate's own cross-section as the flux turns and runs sideways out from under the magnet. For anything being designed rather than sketched, measure the part.
Where the numbers come from
118 grades read straight off Arnold's published demagnetization curves — the curve paths themselves, not the tables — then cross-checked against the Br and Hcj printed on the same page. Remanence, coercivity and knee shape all come from the curves. Temperature behavior is fitted through whatever each sheet supports: two or more points give the quadratic these properties can follow, a single quoted coefficient range gives a straight line, and the page tells you which it did.
Worth knowing before you trust the fourth digit: every neo and GBD curve set turned out to be the room-temperature curve scaled linearly, with a quadratic term under 0.14% of the linear one. The SmCo sheet is the only one here that genuinely bends. The drawn knees are idealized too — they imply an energy product about 5% above what the same page prints.