Permanent magnets have consistent non-varying magnetic fields that have north and south poles associated with them. In most applications, the magnetic polarity has no effect on the applications. The magnets come in different materials – rare earth, Alnico and ferrite. Several different sizes and shapes are offered having different magnetic strengths. The rare earth is the strongest, Alnico is the most stable, and ferrite is the least expensive. The magnetic strength is generally measured in gauss or milliTesla.