A magnetic material turns into a magnet just by sitting in a field

1. Hold the N pole of a magnet just above a steel paper clip and the clip jumps up, but before it can be pulled it has to become a magnet itself.
2. The field of the permanent magnet magnetises the clip so that the end nearest the magnet becomes an S pole, which is why the two facing ends always attract.
3. The far end of the clip is then an N pole, so a second clip hangs off the bottom of the first and a third off the bottom of that.
4. Each clip down the chain is a weaker magnet than the one above it, because the field weakens with distance, so the chain runs out after three or four clips.
5. Lift the permanent magnet away and the chain drops apart, because the induced magnetism in the clips almost entirely disappears.
6. Induction also explains why a magnet never repels an unmagnetised magnetic material: the material always ends up with the opposite pole facing the magnet.

Soft iron makes a temporary magnet that switches off

1. "Soft" here describes the magnetism, not the metal: soft iron is a hard, heavy metal that would hurt if you dropped it on your foot.
2. Placed in a magnetic field, soft iron magnetises strongly and almost instantly.
3. Take the field away and it keeps almost none of that magnetism, so it goes back to being an ordinary lump of iron.
4. That behaviour is exactly what an electromagnet core needs: wind a coil round a soft-iron core, switch the current on and the core boosts the field enormously, switch it off and the load drops.
5. A scrapyard crane lifts a car body with a soft-iron electromagnet and releases it over the crusher by breaking the circuit, which a permanent magnet could never do.

Steel makes a permanent magnet that keeps its magnetism

1. Steel is iron with a small amount of carbon in it, and that small change makes its magnetism hard to alter in either direction.
2. Magnetising a steel bar needs a stronger field, or repeated stroking with a magnet, where soft iron would have magnetised at once.
3. Once it is magnetised, steel holds its poles for years with no power supply, which is why compass needles, fridge magnets and loudspeaker magnets are made from magnetically hard materials.
4. Heating a steel magnet strongly, hammering it, or dropping it repeatedly will knock the magnetism out, because these jolt the internal arrangement out of line.

Pick the material by asking whether the magnetism has to switch off

1. The two materials differ on three properties, and questions usually want them as matched pairs.
   1. How easily they magnetise: soft iron magnetises easily and quickly, steel resists and needs a stronger field.
   2. How much they keep: soft iron retains almost none once the field is removed, steel retains nearly all of it.
   3. What they are used for: soft iron for the cores of electromagnets, relays and transformers, steel for permanent magnets such as compass needles and fridge magnets.
2. If a device needs magnetism you can turn on and off, choose soft iron; if it needs magnetism that stays without any electricity, choose steel.