Permanent magnets have a consistent non-varying magnetic field that has a north and south pole associated with them.The magnets come in different materials – NdFeB,SmCo,Alnico and hard Ferrite.All of them come in different sizes and shapes and have different magnetic strengths. The rare earth magnets are the strongest, Alnico are the most stable, and Ferrite are the cheapest.The magnetic strength is generally measured by gauss meter. Magnets have reversible and irreversible demagnetization specifications. Engineers should consider the shock, vibration, strong external magnetic fields as well as high temperatures in their designs. All these factors influence the magnetic force and the long term stability in different applications.