Field lines and iron filings

A magnet changes the space around it. That change is the magnetic field B. At every point it has a size and a direction, and it is measured in tesla (T). A fridge magnet's field is a few millitesla (mT). Earth's is about 50 microtesla (µT).

We draw the field with field lines:

• the direction of a line is the way an N pole would be pushed;
• outside a magnet, lines leave N and enter S;
• inside the magnet they carry on from S back to N, so every line is a closed loop;
• where lines crowd together, the field is strong.

Each filing becomes a tiny magnet and swings to line up with the field, so together they trace the lines. But a filing has no fixed N end, so filings show the shape of the field, not its direction. For direction you need a compass.

Switch between the 3D and flat views too. The field does not lie flat on the table. It fills the space all around the magnet, above and below.