Monday January 25, 2016 Q&A

Why are submarines demagnetized?


1 So they can dive deeper

2 So they always know which way is north

3 So they can avoid detection

Question related image
Correct answer: so they can avoid detection The decision to demagnetize warships was an indirect result of efforts to detect submarines far below the surface. This was a very important question for the British and American militaries during the Second World War. They needed to find a way to react quickly to enemy attacks by submarines, which were being increasingly used, and to deal with the magnetic mines that the Germans were using to destroy the British fleet. Steel is a magnetic metal, which means it distorts the lines of the earth’s magnetic field in the immediate vicinity. Steel is also a major component of warships and submarines, which therefore behave like magnets when they move, creating a significant anomaly in the earth's magnetic field. US Navy researchers came up with the idea of creating a sensor uniquely capable of detecting these anomalies using a map of the earth’s magnetism. Model of the magnetic anomaly of a submarine on a reference plane * The sensor they developed was the magnetometer. The underlying principle was remarkably simple for the precision that was required. A transformer was made using a ferromagnetic core that is highly sensitive to magnetic fields. Two wire coils are wound around the core. If no external field is applied, the primary coil causes the core to magnetize to saturation in one direction and then the other. The alternating current at the poles of the secondary coil therefore has an average value of zero. When an external field exists, however, the field induced in the core reaches a higher value in one direction than the other, and the current on the secondary coil is no longer symmetric around zero. This change indicates the presence of the external field. Increasingly sensitive magnetometers, along with other detection and signal processing instruments, continue to be developed to this day. Ultra-sensitive magnetometers may be based either on mechanical principles or on an effect of quantum mechanics. But even with a basic magnetometer, engineers at the time were able to accurately map out the earth's magnetic field. As a result, any anomaly observed indicated the presence of a metallic hulk under the water, which had to be a submarine. To this day, anti-submarine aircraft continue to use this type of equipment. During the Second World War, engineers had to figure out how to avoid creating the electromagnetic variation that made ships vulnerable. In other words, they needed to demagnetize them. It took them several months to come up with the answer, which was to send an electric current through the ship to counter the ship’s own magnetic field. The original demagnetization method consisted in installing electromagnetic coils in the ships by stringing an electric cable the length of the ship and pulsing 2,000 amps through the cable. Nowadays, thick copper cables encircle the ship’s hull and superstructure, and a very strong electric current (up to 4,000 amps) runs through the cables. This system effectively neutralizes the ship’s magnetic signature. Thanks to Jean-Philippe Ansermet from EPFL *Yannick Vuillermet. IMMUNISATION EN BOUCLE FERMEE - APPLICATION AU SOUS-MARIN DOUBLE COQUE Identification de l'aimantation à partir de mesures du champ magnétique proche. Sciences de l'ingénieur. Institut National Polytechnique de Grenoble - INPG, 2008.