Every magnet datasheet is written in four letters: H, B, M and J. They look similar but mean very different things - and mixing them up is the most common source of specification mistakes. Here is each one in a sentence.
H - the push
H is the magnetic field produced by a source: a coil, a wire, or another magnet. It describes what is being applied, no matter what material is in the way. Think of it as the "push" from outside.
B - what you feel
B is the total field that a sensor or a piece of iron actually feels - the push from outside plus the material's own response. It is what a gaussmeter reads and what determines pull force. Units: tesla (T) or gauss (G) - remember, 1 T = 10,000 G.
M - the material's answer
M is how strongly the material itself is magnetized - the sum of all the tiny atomic magnets inside, per unit of volume. It is the material's response to the push H.
J - M in tesla clothes
J is simply M converted into the same units as B. Engineers call it polarization or intrinsic induction. The remanence Br on a datasheet - the strength left when the external field is zero - is just J at that point.
| Letter | In one sentence | Unit |
|---|---|---|
| H | The applied push from outside | A/m (Oe) |
| B | The total field you feel | T (G) |
| M | How strongly the material responds | A/m |
| J | M expressed in tesla - intrinsic induction | T (G) |
How engineers use them
Use B for force and flux calculations, H for magnetizing and demagnetizing fields, and Hcj (derived from J) to judge resistance to demagnetization - always check Hcj for thin magnets or hot environments. Full conversion table: magnetic units.
Read on: how grade names use these → · reading a hysteresis loop →