The needle must be magnetized.
Explanation
A compass functions based on the directive property of magnetism. A freely suspended or floating magnet experiences a torque due to the Earth's magnetic field, causing it to align along the geographic North-South direction. This alignment occurs because the Earth behaves like a giant magnet.
Option Analysis:
- The needle must be coated with copper. is Incorrect: Copper is a non-magnetic (specifically diamagnetic) material. Coating a steel needle with copper does not impart the magnetic dipole moment required for the needle to align with the Earth's magnetic field.
- The needle must be magnetized. is Correct: For a steel needle to function as a compass, it must possess magnetic poles (North and South). An unmagnetized piece of steel does not have a net magnetic moment and will not experience the directional torque from the Earth's magnetic field. Therefore, the needle must be magnetized to orient itself North-South.
- The water must be saline. is Incorrect: While saline water has higher density and surface tension, which may aid in floating the needle, the chemical composition of the water does not influence the magnetic interaction. An unmagnetized needle would float randomly regardless of the water's salinity.
- The container must be made of ferromagnetic material. is Incorrect: A container made of ferromagnetic material (like iron) would act as a magnetic shield or attract the needle locally. This would distort the external magnetic field lines, preventing the needle from aligning correctly with the Earth's magnetic field.
Key Takeaway:
A compass is fundamentally a magnet. Any object used as a compass must be magnetized to possess a magnetic moment, allowing it to interact with and align along the Earth's magnetic field lines.