Iron, nickel, and cobalt are common examples of magnetic materials. Air, paper, wood, and plastics are nonmagnetic.
The pole of a magnet that seeks the geographic North Pole of the earth is called a north pole; the opposite pole is a south pole.
Opposite magnetic poles are attracted; similar poles repel. An electromagnet needs current from an external source to provide a magnetic field. Permanent magnets retain their magnetism indefinitely.
Any magnet has an invisible field of force outside the magnet, indicated by magnetic field lines. Their direction is from the north to the south pole.
The open ends of a magnet where it meets a nonmagnetic material provide magnetic poles. At opposite open ends, the poles have opposite polarity.
A magnet with an air gap has opposite poles with magnetic lines of force across the gap. A closed magnetic ring has no poles.
Magnetic induction enables the field of a magnet to induce magnetic poles in a magnetic material without touching.
Permeability is the ability to concentrate magnetic flux. A good magnetic material has high permeability.
Magnetic shielding means isolating a component from a magnetic field. The best shield against a steady magnetic field is a material with high permeability.
The Hall voltage is a small voltage generated across the width of a conductor carrying current through its length, when magnetic flux is applied perpendicular to the current. This effect is generally used in the gaussmeter to measure flux density.
Table 13–1 summarizes the units of magnetic flux Ω and flux density B.
