Question 58

The Correct Answer is C --- ### 2. Explanation of why Option C is correct (The deviation will decrease) The deviation caused by permanent magnetism depends on the relationship between the fixed magnetic force exerted by the ship's permanent magnetism ($P$) and the Earth’s horizontal magnetic directive force ($H$). 1. **Permanent Magnetism ($P$) is Fixed:** The magnetic field generated by the ship’s hard iron (permanent magnetism) is relatively stable and independent of the ship's location (ignoring long-term aging effects). It provides a constant deflecting force ($F_P$) on the compass. 2. **Earth's Directive Force ($H$) Increases:** The magnetic equator is defined as the line where the Earth's vertical magnetic force ($Z$) is zero. As a ship proceeds towards the magnetic equator, the Earth’s horizontal magnetic force ($H$), which acts as the directive force that keeps the compass aligned, increases. 3. **Deviation is a Ratio:** The magnitude of the permanent magnetism deviation ($\delta_P$) is inversely proportional to the horizontal force ($H$): $$\text{Deviation} \propto \frac{\text{Deflecting Force} (F_P)}{\text{Directive Force} (H)}$$ Since $F_P$ remains constant and $H$ increases as the ship approaches the magnetic equator, the ratio decreases. Consequently, the uncorrected deviation due to permanent magnetism decreases. ### 3. Explanation of why the other options are incorrect **A) It remains the same** This is incorrect because the Earth's magnetic field components ($H$ and $Z$) change significantly with latitude. Since deviation is highly dependent on the horizontal component ($H$), the deviation due to permanent magnetism cannot remain the same as the ship changes its latitude relative to the magnetic equator. **B) It is unimportant and may be neglected** This is incorrect. While the deviation due to permanent magnetism is relatively smaller near the magnetic equator than near the magnetic poles (where $H$ is weaker), it is still a measurable error that must be accounted for and corrected, especially if the permanent magnetism component is large. Deviation cannot be generally neglected. **D) The deviation will increase** This is incorrect. Deviation due to permanent magnetism increases only when the Earth's horizontal directive force ($H$) decreases. This happens when the ship moves *away* from the magnetic equator towards a magnetic pole. If the ship moves *towards* the equator, $H$ increases, and deviation decreases. (Note: Induced magnetism deviation, specifically the deviation related to coefficient $E$, can increase near the equator, but the question specifically refers to permanent magnetism.)