A long, straight wire carries a current i. The magnetising 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 at P will ______________ .

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remain almost constant From the Biot-Savart law, magnetic field (B) at a point P close to the wire carrying current i is given by, \overrightarrow{B} = \mu_0/4$\pi$\frac{i \overrightarrow{dl} $\times$ \overrightarrow{r}}{r^3} Magnetising field intensity (H) will be, H = B/\mu_0 = $\frac{1}{4}$$\pi$\frac{i \overrightarrow{dl} $\times$ \overrightarrow{r}}{r^3} Now, as the cylindrical rod is brought close the wire such that centre of the rod is at P, then distance of point P from the wire(r) will remain same. Hence, magnetic field intensity will remain almost constant. Also even when the rod is carrying any current then B will be zero at the centre of the rod so the value of Magnetising field intensity will remain the same at point P.

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