Marine Magnetic Compass adjustment:magnetic induction and permeability;

Excellent question. Understanding magnetic induction and permeability is fundamental to grasping why a marine magnetic compass needs adjustment and how that adjustment is performed.

Let’s break it down, starting with the core physics and then applying it directly to the ship and its compass.

1. The Core Physics: Induction and Permeability

Magnetic Induction

Magnetic induction is the process by which a material becomes magnetized when it is placed in a magnetic field.

• The Earth’s Field: The Earth itself is a giant magnet, creating a magnetic field that surrounds it. This field is what a perfect compass would point to (Magnetic North).
• The Process: When you place a piece of magnetizable material (like iron or steel) within this field, the Earth’s magnetic field lines pass through it. This external field realigns the magnetic domains within the material, causing the material itself to become a magnet.
• Simple Analogy: Think of a strong permanent magnet and a paperclip. The paperclip isn’t normally magnetic. But if you bring the magnet close to it, the paperclip becomes magnetized by induction and can then pick up other paperclips. When you take the strong magnet away, the paperclip loses most (but not all) of its magnetism.

Permeability (μ)

Permeability is a measure of how easily a material can be magnetized. It quantifies how well a material can support the formation of a magnetic field within itself.

• High Permeability: Materials like soft iron and steel have high permeability. They offer a very easy path for magnetic field lines. They essentially “suck in” or concentrate the magnetic field lines. This is why they are so easily and strongly magnetized by induction.
• Low Permeability: Materials like air, wood, aluminum, or a vacuum have very low permeability. Magnetic field lines pass through them, but they don’t concentrate the lines or become strongly magnetized.

The Key Connection: A ship is a massive object made of steel. Because steel has high permeability, it is very susceptible to magnetic induction from the Earth’s magnetic field.

2. How This Creates a Problem on a Ship

The ship itself becomes a large, complex magnet due to induction. This ship-based magnetism interferes with the compass, causing it to point away from Magnetic North. This error is called Deviation.

The ship’s magnetism is divided into two types:

A. Permanent Magnetism (“Hard Iron”)

This is magnetism that is “locked into” the ship’s structure. It is acquired during construction.

• Cause: While the ship is being built, the steel plates are hammered, riveted, welded, and vibrated. All this happens while the ship is sitting stationary in the Earth’s magnetic field. This process “fixes” the magnetic alignment in the steel, turning the hull into a giant permanent magnet.
• Characteristics:
  • Its strength and polarity are constant.
  • It is fixed relative to the ship’s structure.
  • As the ship turns, this fixed magnetic field turns with it, deflecting the compass needle differently on various headings. This causes semicircular deviation (it’s maximum on two headings and zero on two others).

B. Induced Magnetism (“Soft Iron”)

This is temporary magnetism that is induced into the ship’s soft iron components by the Earth’s magnetic field. This is the direct result of permeability and induction in real-time.

• Cause: The ship’s steel has high permeability. As the ship sails, the Earth’s magnetic field continuously induces magnetism into its structure.
• Characteristics:
  • The polarity and strength of this magnetism are not fixed relative to the ship.
  • It changes depending on the ship’s heading and its location on Earth (magnetic latitude).
  • Its polarity is always aligned with the Earth’s magnetic field at that moment.

Example of Induced Magnetism in Action: Imagine a long steel deck beam running fore-and-aft.

• When the ship is heading North, the North-seeking pole of the Earth’s field induces a South pole in the forward end of the beam and a North pole in the aft end.
• When the ship turns and heads South, the induction reverses. The forward end of the beam now becomes a North pole and the aft end a South pole.

This constantly changing induced magnetism causes quadrantal deviation (it’s maximum on the four intercardinal headings: NE, SE, SW, NW, and zero on the cardinal headings: N, E, S, W).

3. The Solution: Compass Adjustment (Applying the Principles)

Compass adjustment (or “swinging the compass”) is the process of counteracting the ship’s magnetism by placing small correctors near the compass. We fight fire with fire, using the same principles of magnetism.

Here’s how permeability and induction relate to the correctors:

Correcting Permanent Magnetism (Hard Iron)

To cancel out the ship’s permanent magnetic field, we use other permanent magnets.

• B Corrector: A set of small, fore-and-aft magnets to counteract the ship’s fore-and-aft permanent magnetism.
• C Corrector: A set of small, athwartships (port-starboard) magnets to counteract the ship’s athwartships permanent magnetism.
• Heeling Magnet: A vertical magnet placed directly under the compass to correct for errors that appear when the ship rolls or heels.

Correcting Induced Magnetism (Soft Iron)

To cancel out the ship’s induced magnetism, we use pieces of high-permeability soft iron. We place them so they acquire an induced magnetism that is equal and opposite to the ship’s unwanted induced magnetism.

• Quadrantal Correctors (Soft Iron Spheres): These are two spheres of soft iron placed on either side of the compass (port and starboard). When the ship is on an intercardinal heading (e.g., NE), the ship’s horizontal steel induces a magnetic field that deflects the compass. The spheres, because of their high permeability, also become magnetized by induction. They are positioned so that the field they create is equal and opposite to the unwanted field from the ship’s structure, canceling it out.

• Flinders Bar: This is a vertical bar of soft iron placed in a tube at the front or back of the binnacle. It corrects for induced magnetism in the ship’s vertical steel. This type of error is most noticeable when a ship travels through large changes in magnetic latitude (e.g., from the equator towards a pole). The Flinders Bar acquires a vertical induced magnetism that is opposite to that of the ship’s vertical structures, providing the necessary correction.

Summary Table

In short, the high permeability of a ship’s steel makes it a prime candidate for magnetic induction, which creates the compass deviation. The adjustment process cleverly uses the very same principles to cancel out the errors.