<P> For a closed magnetic circuit (no air gap), such as would be found in an electromagnet lifting a piece of iron bridged across its poles, equation (1) becomes: </P> <Dl> <Dd> B = N I μ L (3) (\ displaystyle B = (\ frac (NI \ mu) (L)) \ qquad \ qquad \ qquad \ qquad \ qquad \ qquad (3) \,) </Dd> </Dl> <Dd> B = N I μ L (3) (\ displaystyle B = (\ frac (NI \ mu) (L)) \ qquad \ qquad \ qquad \ qquad \ qquad \ qquad (3) \,) </Dd> <P> Substituting into (2), the force is: </P>

Which describes the requirements for making an electromagnet