Marine Magnetic Compass construction and siting :Magnetic screening;

Of course. This is an excellent topic covering the core principles of ensuring a magnetic compass remains a reliable tool for navigation at sea. Let’s break down the construction, siting, and the crucial concept of magnetic screening and correction.

1. Construction of a Marine Magnetic Compass

A marine magnetic compass is not just a simple needle pointing north. It’s a robust, precision instrument designed to function accurately despite the violent motion of a ship at sea.

Key Components:

• Compass Bowl: The main housing, made of a non-magnetic material like brass, bronze, or reinforced fiberglass. It is filled with a liquid to dampen the motion of the compass card.
• The Liquid: A mixture of distilled water and pure ethyl alcohol is typically used. The alcohol prevents freezing, and the water provides ideal density. The liquid serves two purposes:
  1. Damping: It slows the oscillation of the compass card, making it readable in rough seas.
  2. Buoyancy: It reduces the weight on the pivot, minimizing friction and wear.
• Compass Card: A lightweight, non-magnetic card marked with 360 degrees and the cardinal points (N, S, E, W).
• Magnetic Element: Attached to the underside of the card are two or more powerful, parallel magnetic needles or bar magnets. Using multiple magnets creates a stronger magnetic moment, making the compass more stable and directive.
• Pivot and Jewel: The card assembly is balanced on a very sharp, hardened pivot point (often made of iridium). This pivot rests in a jeweled cup (often a synthetic sapphire) to ensure minimal friction, allowing the card to turn freely.
• Lubber’s Line: A fixed vertical line inside the compass bowl that is perfectly aligned with the ship’s fore-and-aft line (the bow). The helmsman steers the ship by keeping the desired course on the compass card aligned with the lubber’s line.
• Gimbals: This is the most critical feature for a marine compass. The compass bowl is mounted in a set of two concentric rings (gimbals) that pivot at right angles to each other. This system allows the compass bowl to remain horizontal, regardless of the ship’s rolling (side-to-side) and pitching (fore-and-aft) motion.
• Expansion Bellows: A small, flexible diaphragm or bellows is built into the bowl to accommodate the expansion and contraction of the liquid due to temperature changes, preventing bubbles or pressure build-up.