Magnetic lines of force

Magnetic lines of force can be made visible using an iron powder. In a magnetic field, each iron particle gets magnetized along the direction of the field. Two nearby particles aligned with the field are then attracted while if they are aligned perpendicularly to the field, they are repelled from one another. This results in a tendency of the powder grains to align into chains and rows along the field lines, and neighboring chains repel one another. This way the magnetic field is visualized beautifully. The first row of pictures shows the field near one pole of a magnet and the field around a cylindrical ferrite magnet. Note the lines of force emerging from the winding of the screw. The first picture in the second row shows the field around a syringe into which a strong NdFeB magnet was inserted. The last row shows the field of a ring magnet (note the absence of the iron near the axis where the field is almost zero) and the field near four magnets with alternating poles.