Terrestrial and ship's magnetism :effects of permanent and induced magnetism;

Of course. This is a fundamental topic in marine navigation. Here is a detailed explanation of terrestrial and ship’s magnetism, breaking down the effects of permanent and induced magnetism.

Overview

A magnetic compass works by aligning itself with the Earth’s magnetic field lines. On land, this is relatively straightforward. However, a ship, being a massive structure made primarily of steel, develops its own magnetic field. This shipboard magnetism interferes with the Earth’s field, causing the compass to point away from Magnetic North. This error is called Deviation.

To understand this error, we must first understand the two sources of magnetism involved: the Earth’s (terrestrial) and the ship’s.

Part 1: Terrestrial Magnetism (The Earth’s Magnetic Field)

The Earth acts like a giant, slightly weak magnet. This is believed to be generated by the motion of molten iron in its outer core.

Key Characteristics:

1. Magnetic Poles: The Earth has a North and a South Magnetic Pole. These are not in the same location as the geographic poles (the axis of Earth’s rotation).
2. Magnetic Field Lines: Invisible lines of force flow from the South Magnetic Pole to the North Magnetic Pole. A magnetic compass needle is designed to align itself horizontally with these lines.
3. Variation (or Declination): Because the magnetic and geographic poles are in different places, there is an angular difference between True North and Magnetic North at most locations on Earth. This angle is called Variation. It is recorded on nautical charts for a specific area and changes slowly over time.
4. Inclination (or Dip): The Earth’s magnetic field lines are only parallel to the surface at the magnetic equator. As you move towards the poles, they dip downwards, becoming vertical at the magnetic poles. This vertical angle is called Dip or Inclination. This concept is crucial for understanding induced magnetism.