Q.Which property of bar magnet is used in navigation?

The retentivity and coercivity of a ferromagnetic material -- how much magnetization it retains, and how hard it is to demagnetise -- decide which of two very different devices it is suited to build. An electromagnet uses a SOFT ferromagnetic core (large permeability, low retentivity and coercivity -- a thin hysteresis loop) inside a solenoid: passing current through the solenoid magnetises the core, boosting the field a thousand-fold or more, while switching the current off lets the field collapse to near zero, since the soft core retains almost none of its magnetisation on its own. This switchability makes electromagnets useful in electric bells, loudspeakers, circuit breakers, cranes for lifting iron loads, and (as superconducting magnets, for the highest fields) in NMR spectroscopy. …