<P> The strength of a given magnet is sometimes given in terms of its pull force--its ability to move (push / pull) other objects . The pull force exerted by either an electromagnet or a permanent magnet at the "air gap" (i.e., the point in space where the magnet ends) is given by the Maxwell equation: </P> <Dl> <Dd> F = B 2 A 2 μ 0 (\ displaystyle F = ((B ^ (2) A) \ over (2 \ mu _ (0)))), </Dd> </Dl> <Dd> F = B 2 A 2 μ 0 (\ displaystyle F = ((B ^ (2) A) \ over (2 \ mu _ (0)))), </Dd> <Dl> <Dd> F is force (SI unit: newton) </Dd> <Dd> A is the cross section of the area of the pole in square meters </Dd> <Dd> B is the magnetic induction exerted by the magnet </Dd> </Dl>

Name the substances from which artificial magnets are made