Bar Magnet

The lines of magnetic field from a bar magnet form closed lines. By convention, the field direction is taken to be outward from the North pole and in to the South pole of the magnet. Permanent magnets can be made from ferromagnetic materials.

An iron core has the effect of multiplying greatly the magnetic field of a solenoid compared to the air core solenoid on the left.

Electromagnet

Electromagnets are usually in the form of iron core solenoids. The ferromagnetic property of the iron core causes the internal magnetic domains of the iron to line up with the smaller driving magnetic field produced by the current in the solenoid. The effect is the multiplication of the magnetic field by factors of tens to even thousands. The solenoid field relationship is

and k is the relative permeability of the iron, shows the magnifying effect of the iron core.