Steel ships and submarines can become magnetic because iron in their hulls lines up with Earth’s magnetic field. This creates a magnetic signature, which is a pattern that can be detected by sensors in the ocean. Magnetic naval mines use this idea by sensing changes in the local magnetic field when a large metal vessel passes nearby.
Degaussing matters because it helps hide the vessel’s magnetic signature and lowers the chance of triggering these mines.
Degaussing uses electric coils installed inside or around the hull to create a controlled magnetic field. The coil field is adjusted so it partly cancels the ship’s own magnetic field, making the total field near the vessel much smaller. The current must be tuned for the ship’s size, shape, heading, location, and amount of magnetization.
Modern systems can change coil currents automatically as a ship moves through different magnetic environments.
Key Facts
- A steel hull can become magnetized when its magnetic domains align with Earth’s magnetic field.
- Degaussing reduces a ship’s magnetic signature by using electric current in coils to create an opposing magnetic field.
- Total magnetic field near the ship can be modeled as Btotal = Bship + Bcoil.
- Best cancellation occurs when Bcoil is nearly equal in strength and opposite in direction to Bship.
- For a simple coil, magnetic field strength increases with current, so B is proportional to I.
- Magnetic mines can be triggered when a sensor detects a field change above a set threshold, such as ΔB > Bthreshold.
Vocabulary
- Degaussing
- Degaussing is the process of reducing or canceling unwanted magnetism in a ship or object.
- Magnetic signature
- A magnetic signature is the measurable pattern of magnetic field produced by an object such as a ship.
- Degaussing coil
- A degaussing coil is an electric wire loop or cable system that produces a magnetic field when current flows through it.
- Magnetic field
- A magnetic field is the region around a magnet or electric current where magnetic forces can act.
- Magnetic mine
- A magnetic mine is an underwater explosive device that can detect changes in magnetic field caused by nearby metal vessels.
Common Mistakes to Avoid
- Thinking degaussing makes a ship invisible to all sensors. It only reduces the magnetic signature, not sound, radar, sonar, heat, or visual detection.
- Assuming the coil current is always the same. The needed current changes with the ship’s heading, location, hull magnetization, and Earth’s magnetic field.
- Drawing the coil field in the same direction as the ship’s magnetic field. That would add to the magnetic signature instead of canceling it.
- Forgetting that cancellation is usually partial. Real ships have complex shapes and nonuniform magnetization, so degaussing lowers the field rather than making it exactly zero everywhere.
Practice Questions
- 1 A ship produces a magnetic field of 18 microtesla at a mine sensor. Its degaussing coils produce an opposing field of 14 microtesla at the same point. Using Btotal = Bship + Bcoil, where the coil field is negative, what is the remaining field change?
- 2 A mine triggers if the magnetic field change is greater than 6 microtesla. A ship has a magnetic signature of 22 microtesla at the mine location. What opposing coil field magnitude is needed to reduce the remaining field to 6 microtesla?
- 3 Explain why a degaussing system must be adjusted when a ship turns from traveling north to traveling east, even if the ship’s speed does not change.