A magnetic field is an invisible area of force that surrounds a magnet or anything that carries electric current. You can't see a magnetic field, but you can observe its effects when it pulls a paper clip toward a refrigerator magnet or makes a compass needle line up in a certain direction.

Every magnet creates a magnetic field around itself. The field is strongest near the magnet's poles (its ends) and gets weaker as you move away. When you hold two magnets close together, you can feel their magnetic fields pushing or pulling before the magnets even touch. If you sprinkle iron filings around a magnet, they arrange themselves in curved lines showing the shape of the invisible field.

Earth itself has a giant magnetic field, generated by electric currents in its molten iron core. This planetary magnetic field acts like a protective shield, deflecting harmful particles from the sun. It also makes compasses work: the needle aligns itself with Earth's magnetic field, generally pointing toward Earth's magnetic north.

Scientists often visualize magnetic fields using field lines, imaginary lines showing the direction the magnetic force points. These lines are drawn from a magnet's north pole to its south pole. Where the lines are closer together, the field is stronger. Understanding magnetic fields helps engineers design everything from electric motors to MRI machines to the hard drives that store computer data.