Magnetic Compass

Revision Notes on Magnetic Compasses

What They Are

A magnetic compass is a navigational instrument that shows direction relative to the Earth's magnetic poles. It is an essential tool for maritime navigation, used extensively on ships to determine heading and course.

How They Work

The magnetic compass operates based on the Earth's magnetic field. It consists of a magnetized needle or card, which aligns itself with the Earth's magnetic lines of force. The needle points toward the magnetic north pole, allowing navigators to determine their direction.

Components:

  • Needle/Card: Magnetized to align with the magnetic field.
  • Pivot: The point on which the needle or card rotates.
  • Damping Fluid: Used to stabilize the needle, preventing oscillation.
  • Compass Rose: Marked with degrees or points for reference.

Use in Navigation

  • Determining Course: Helps in plotting and maintaining a course.
  • Dead Reckoning: Assists in estimating current position based on previous positions.
  • Bearing: Used to take bearings on landmarks or celestial bodies.
  • Course Correction: Vital for making adjustments to stay on the correct route.

Compass Error Calculation

Compass error is the difference between the true direction and the direction indicated by the compass. It includes variation and deviation:

  1. Variation: The difference between true north (geographic) and magnetic north. It varies with geographic location and is indicated on nautical charts.
  2. Deviation: Caused by magnetic influences within the vessel itself. It varies with the vessel's heading.

Calculating Compass Error:

Compass Error=True Bearing−Compass Bearing\text{Compass Error}

To find:

  1. Take a Bearing: Measure the bearing of a known object using the compass.
  2. Check the Chart: Find the true bearing of the same object from your current position on the nautical chart.
  3. Apply Variation: Adjust the true bearing for the local variation found on the chart.
  4. Subtract Compass Bearing: The difference gives you the compass error (including deviation).

Practical Tips for Using Magnetic Compasses

Regular Checks and Maintenance

  • Frequent Monitoring: Regularly check the compass to ensure it is functioning correctly. Any erratic movements or sticking of the compass card should be investigated immediately.
  • Keep It Clean: Ensure that the compass housing is clean and free from debris, which can interfere with the needle's movement.

Minimize Magnetic Interference

  • Avoid Magnetic Disturbances: Keep electronic devices, metal objects, and any magnetic materials away from the compass to avoid interference.
  • Positioning: Place the compass in an area of the vessel where it is least likely to be influenced by other magnetic fields.

Calibration and Adjustment

  • Swinging the Compass: Perform regular compass swings to calibrate and correct deviations. This should be done especially after any significant repairs or alterations to the vessel.
  • Adjust Correctors: Use the correctors like the Flinders bar, heeling error magnets, and quadrantal spheres appropriately to counteract deviations from the Earth's magnetic field.

Navigational Procedures

  • Cross-Check Bearings: Always cross-check compass bearings with other navigational aids such as GPS or celestial navigation to ensure accuracy.
  • Plot Regularly: Plot your position regularly on the chart to monitor your progress and ensure you are on course. This helps in identifying any compass errors early.

Hemisphere Changes

  • Adjust Flinders Bar: When changing hemispheres, remember to adjust the Flinders bar to account for differences in magnetic inclination.
  • Recalibrate: After significant latitude changes, recalibrate your compass to correct for any new deviations.

Safety Measures

  • Night Navigation: Ensure your compass is illuminated or easily readable at night. Many modern compasses have built-in lighting for this purpose.
  • Backup Compass: Always have a backup magnetic compass available in case the primary compass fails.

Error Awareness

  • Understand Variation and Deviation: Be familiar with the local magnetic variation (difference between true north and magnetic north) and deviation (error due to the ship's magnetism) for accurate navigation.
  • Keep a Deviation Card: Maintain an up-to-date deviation card that lists the known compass deviations for various headings. Use this card to correct your compass readings.

Magnetism in Compasses

Permanent Magnetism:

  • Definition: Magnetism that the ship acquires and retains due to the processes of construction, welding, and the Earth's magnetic field.
  • Characteristics: Permanent magnets influence the compass consistently, regardless of the heading.
  • Correction: Requires placement of permanent correctors (e.g., small magnets) around the compass to counteract the magnetic field.

Semi-Permanent Magnetism:

  • Definition: Magnetism that varies depending on the ship's heading and environmental factors. It can be influenced by changes in latitude, temperature, and time.
  • Characteristics: Semi-permanent magnetism can alter over time and with movement of the ship.
  • Correction: Adjustments are made with correctors that can be fine-tuned, such as soft iron correctors or small movable magnets.

Procedure for Correcting a Magnetic Compass

Who Can Do It:

  • Qualified Personnel: The correction of a magnetic compass should be carried out by a qualified compass adjuster or a marine surveyor. These professionals are trained to handle the complexities of magnetic influences and correct the compass accurately.

When to Do It:

  • After Construction: When a new vessel is constructed.
  • Following Alterations: After structural changes or installation of new equipment.
  • Periodic Checks: Regularly, as part of routine maintenance.
  • When Errors are Detected: If significant deviations are noticed during navigation.

General Principles (Heading North to South):

  • Swinging the Compass: This is a procedure where the ship is swung slowly in a full circle while taking bearings on known fixed objects at various headings. This helps to identify the deviation at different headings.
  • Adjusting Correctors: By adjusting the correctors (small magnets and soft iron correctors placed around the compass), the deviation can be minimized.

Steps to Correcting Compass:

  1. Preparation: Select a calm day with minimal sea movement. Ensure the vessel is away from magnetic influences such as large metal structures or other ships.
  2. Select Bearings: Identify fixed points for bearings.
  3. Take Bearings: Measure the compass bearing of the identified fixed points as the vessel turns.
  4. Plot Deviation: Plot the observed compass errors at each heading to determine deviation.
  5. Adjust Correctors: Adjust the magnetic correctors (both permanent and adjustable) around the compass to reduce the deviation.
  6. Recheck: Repeat the process to ensure that the errors are minimized. Document the remaining deviation for reference during navigation.

Correctors Used:

  • Flinders Bar: A vertical soft iron bar placed near the compass to correct for the ship's induced magnetism.
  • Heeling Error Magnet: Corrects errors when the vessel heels.
  • Quadrantal Spheres: Soft iron balls placed on either side of the compass to correct for quadrantal deviation.
  • Fore-and-Aft / Athwartship Magnets: Used to balance out errors caused by magnetic fields running lengthwise or crosswise on the ship.

Revision Notes on Magnetic Compasses - Extended with Correctors

Procedure for Correcting a Magnetic Compass: Correction of a magnetic compass involves the use of various correctors to counteract different influences of the Earth's magnetic field. Here's a detailed look at how correctors like the Flinders bar, heeling error magnet, and quadrantal spheres work to address specific magnetic deviations:

  1. B Coefficient (Semicircular Deviation):
    • Fore-and-Aft Magnets: These magnets are placed along the fore-and-aft line of the ship and correct errors that cause the compass needle to swing in a predictable manner as the ship turns. They counteract the semicircular deviation caused by permanent and induced magnetism in the ship’s fore-and-aft direction.
  2. C Coefficient (Semicircular Deviation):
    • Athwartship Magnets: Positioned across the ship (athwartship), these correctors deal with the horizontal magnetic field components that affect the compass. They help in adjusting the semicircular deviation caused by the ship's magnetism in the athwartship direction.
  3. D Coefficient (Quadrantal Deviation):
    • Quadrantal Spheres: These soft iron balls are placed on either side of the compass and are essential for correcting quadrantal deviations. These deviations occur due to the interaction of the ship's magnetic field with the Earth’s magnetic field at different headings and are more prominent when the ship is at 45° angles to the cardinal points.
  4. Heeling Error:
    • Heeling Error Magnet: This is a vertical corrector placed below the compass to mitigate errors caused by the ship's heeling. When a ship tilts due to wave action or when turning, the vertical component of the ship’s magnetism can affect the compass reading, and this corrector compensates for such changes.
  5. Flinders Bar:
    • Vertical Soft Iron Bar: Placed near the compass to correct for induced magnetism in the vertical plane. This corrector is vital when the ship changes latitude, as the Earth’s magnetic inclination varies with latitude, influencing the vertical component of the magnetic field.

By systematically applying and adjusting these correctors, a qualified compass adjuster can significantly reduce the errors in a magnetic compass, ensuring accurate navigation. The process involves careful measurement and correction to account for the various magnetic influences acting upon the compass.

 

Adjusting the Flinders Bar When Changing Hemisphere

When changing hemispheres, the Earth's magnetic field affects the ship's magnetic compass differently due to variations in the magnetic inclination (the angle between the Earth's magnetic field and the horizontal plane). This is where the Flinders bar comes into play:

Understanding Magnetic Inclination:

    • In the Northern Hemisphere, the Earth's magnetic field has a downward inclination, meaning the magnetic field lines tilt downward.
    • In the Southern Hemisphere, the inclination is upward, with the magnetic field lines tilting upward.
  2. Purpose of the Flinders Bar:
    • The Flinders bar is a vertical soft iron bar placed near the compass to correct for the ship's induced magnetism in the vertical plane. It helps in counteracting the effects of the Earth's magnetic inclination.
  3. Adjustment Procedure:
    • Assess the Change: When the ship crosses the equator or moves significantly in latitude, assess the need to adjust the Flinders bar based on the change in magnetic inclination.
    • Reposition the Bar: Depending on the direction of travel (north to south or vice versa), the Flinders bar may need to be moved up or down to counteract the changes in the vertical magnetic component.
    • Recalibrate: After adjusting the Flinders bar, perform a new compass swing to measure and correct for any new deviations introduced by the change in magnetic field inclination.
  4. Practical Tips:
    • North to South Movement: As the ship moves southward (toward the Southern Hemisphere), the Flinders bar might need to be raised slightly to account for the increasing upward magnetic inclination.
    • South to North Movement: Conversely, when moving northward (toward the Northern Hemisphere), the Flinders bar might need to be lowered slightly to compensate for the downward magnetic inclination.