T_o is time period of oscillation of the needle in uniform horizontal magnetic field (B_H) of the earth. If another magnetic field is applied perpendicular to B_H such that the needle deflects by $\mathrm{\theta }$ for its equilibrium. Now period of oscillation of the needle is T. The relation between T and T_o is :

• A

  ${\mathrm{T}}^2={{\mathrm{T}}_0}^2\mathrm{cos\theta }$

• B

  $\mathrm{T}={\mathrm{T}}_0\mathrm{cos\theta }$

• C

  $\mathrm{T}=\frac{{\mathrm{T}}_0}{\cos \mathrm{\theta }}$

• D

  ${\mathrm{T}}^2=\frac{{{\mathrm{T}}_0}^2}{\mathrm{cos\theta }}$