The region where the magnetic forces act is called the “magnetic field”
EARTH’S MAGNETISM The Earth itself has a huge magnetic field - as if it had a huge bar magnet embedded at its centre. The Earth’s magnetic field lines emerge from near the geographical north pole and re-enter it at the south pole. The nature of the field around the Earth varies in both strength and direction. The Earth’s magnetic field is strongest at the magnetic poles and weakest near the Equator.
Why does the Earth have a magnetic field? The Earth has, at its centre, a dense liquid core, of about half the radius of the Earth, with a solid inner core. This core is though to be mostly made of molten iron and nickel perhaps mixed with some lighter elements. Circulating ions of iron and nickel in highly conducting liquid region of earth’s core might be forming current loops and producing earth’s magnetism.
The metals affected by magnetism consist of tiny regions called 'Domains' which behave like tiny magnets. Normally they are arranged in the magnetic material all pointing in different directions in a completely random fashion and so their magnetic effects cancel each other out. If an object is magnetized it is because the domains are all made to point in the same direction. This can be done by stroking the magnetic material with a magnet (or magnets) as shown in the diagram. When aligned the domains reinforce one another and create north and south poles at either end.
The relationship between magnetic field strength (H) and magnetic flux density (B) will follow a curve up to a point where further increases in magnetic field strength will result in no further change in flux density. This condition is called magnetic saturation till point (a)
the plotted relationship will follow a different curve back towards zero field strength at which point it will be offset from the original curve by an amount called the remanent flux density or Retentity as shown in graph at point (b)
The 'thickness' of the middle, describes the amount of hysteresis, related to the coercivity of the material as from (c) to (f)