$(a)$ In a pattern of magnetic field lines due to a bar magnet,how can the regions of relative strength be identified?
$(b)$ Compare the strength of the magnetic field near the poles and the middle of a bar magnet.

(N/A) The relative strength of the magnetic field is indicated by the degree of closeness of the magnetic field lines. Where the field lines are crowded,the magnetic field is strong,and where they are far apart,the magnetic field is weak.
$(b)$ The strength of the magnetic field is highest near the poles of a bar magnet because the field lines are most concentrated there. Conversely,the strength of the magnetic field is lowest in the middle of the bar magnet,where the field lines are relatively far apart.