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.