Switchable magnets; holding magnets

A switchable magnet is a simpler type of variable flux source. Magnetic holding devices have a piece of ferrous metal in contact with a magnet. The working point shifts from the open circuit point to the remanence point where H = 0 as the circuit is closed. The maximum force f that can be exerted at the face of a magnet may be derived by considering a toroid which is cut into two C-shaped segments that are separated by a small distance d. The energy appearing in the air gaps is 2 × 1/2μ₀H²_gA_gd = B²_gA_gd/μ₀. The work done separating the segments is 2fd, hence the magnitude of the force per unit area is

When the gap is narrow, B_g = B_m so forces of up to 40 N cm⁻² can be attained for B_m = 1 T. If large blocks of rare-earth magnet are allowed to come into contact, they are very difficult to separate!