A compass needle made of paramagnetic alloy is used at a location where the angle of dip is w/3. When the alloy is slightly cooled below the room temperature, the needle's vertical deflection increases, even though the Earth's magnetic field remains unchanged. Which one of the following best explains this observation?

The observation is explained by Curie's Law, which states that the magnetic susceptibility (χ) of a paramagnetic material is inversely proportional to its absolute temperature (T), expressed as χ = C/T. When the paramagnetic alloy needle is cooled, its susceptibility increases, leading to a stronger induced magnetic moment in the presence of the Earth's magnetic field. This results in a greater magnetic torque acting on the needle, causing it to align more significantly with the Earth's magnetic field lines and thus increasing the vertical deflection.

• Option A is incorrect because changes in the moment of inertia due to thermal contraction are negligible in this context.
• Option C is incorrect because the Earth's dip angle is a property of the magnetic field at a specific location and does not change with the temperature of the needle.
• Option D is incorrect because "slight cooling" is generally insufficient to reach the Curie temperature required for a paramagnetic-to-ferromagnetic phase transition.