Two magnets of the same grade can behave very differently. A thick rod holds its strength; a thin, flat disc quietly gets weaker in a warm room. The material is identical - what differs is shape, described by the permeance coefficient Pc.
Pc in plain words
Pc is a simple "shape score" for how well a magnet resists demagnetizing itself. Every magnet has a north and a south pole that pull on each other through the inside of the magnet, against its own magnetization. The closer the poles, the stronger the opposing effect.
The rule of thumb: long and thin in the magnetized direction = high Pc = hard to demagnetize. Short and flat = low Pc = easy to demagnetize.
Why flat magnets are fragile
A flat disc has its poles almost touching, so the internal opposing field is strong. Its working state sits low on the demagnetization curve, close to the "knee". At that spot, mild heat, a knock, or an opposing field pushes it over the edge - and the loss is permanent.
A slender rod keeps its poles far apart, its working state stays high and safe, and it shrugs off the same disturbances.
Typical values
| Shape | Pc (approx.) | Demag. resistance |
|---|---|---|
| Flat disc (D12 × 1 mm) | < 0.2 | Very low - fails easily |
| Block, 10 × 10 × 6.5 mm | ≈ 1 | Moderate |
| Rod, L/D ≈ 2 | ≈ 5 | Very good |
Approximate values for magnetization along the axis.
What to do with it
Shape and grade work together - motor magnets pair high-Pc shapes with high-coercivity grades (45UH, 48UH, 50UH), and a thin "N50H" disc can still fail at 60 °C. If your part is flat or runs hot, check the shape first.
Read on: closing the magnetic circuit → · magnets and heat →