A long, straight wire carries a current i. The magnetizing field intensity H is measured at a point P close to the wire. A long, cylindrical iron rod is brought close to the wire so that the point P is at the centre of the rod. The value of H and P will
When there is no iron rod the magnetic field at point P close to the wire is given by
The magnetic intensity H is then given by
Where
i=current through the wire
r=distance of point from the wire
Now as the wire and the iron rod are long and we are interested in magnetic intensity at the point P which is at centre of rod, the end effects of a magnetized material are neglected and the magnetic intensity due to magnetization is zero.
There is no effect of rod in magnetic intensity at the centre. The magnetic intensity in a material is then determined by external sources only. Its value in both the cases remains almost constant.
Therefore, value of magnetic intensity(H) at P remains constant.