A thin insulated tightly wound wire forms a plane spiral of $\text{N}$ turns and carries current $\text{I}$. The radii of inside and outside turns are $\text{a}$ and $\text{3a}$ respectively. Then magnetic field at the centre of spiral is $\frac{{\text{μ}}_{\text{0}}\text{NI}}{n\mathrm{a}}\mathrm{In}\left(3\right)$ . Value of $n$ is