Q: A compass needle of magnetic moment 60 A-m2, pointing towards geographical north at a certain place where the horizontal component of earth’s magnetic field is 40 μwb/m2 experience a torque of 1.2 × 10-3 Nm. Find the declination at that place.
Sol: If θ is the declination of the place, then the torque acting on the needle is
τ = M BH sin θ
sin θ = τ/MBH
sin θ = (1.2 ×10-3)/(60 × 40×10-6)
sin θ = 1/2 ∴ θ = 30°