Degaussing refers to the process of neutralising the magnetic field around an object. The process can be applied to any magnetic material, but originated during World War II when then Commander Charles F Goodeve RNVR developed the process as a countermeasure against Germany's magnetic mines, deployed against the British fleet.
The magnetic mine contained a sensitive magnetic sensor, calibrated in milligauss. One milligauss is one thousandth of a gauss, the measurement unit of a magnetic field, named after the German mathematician and physicist Carl Friedrich Gauss. These highly sensitive detectors could detect the passage of a ship above them as it concentrated and distorted the earth's magnetic field, and they were set to trigger the mine when the signal reached its maximum, indicating the ship was at its closest approach. Unlike contact mines, which detonate against the target with the intention of causing a hole, the magnetic mine is a proximity device, intended to disable or destroy its target through the distortion caused by the blast or shock wave generated by its explosion.
Magnetic hysteresis means that degaussing is not generally able to reduce the magnetic field around an object to zero, nor would this be desirable as the mere presence of a magnetic material in the earth's magnetic field will distort and concentrate it. Goodeve's system was developed to produce a known residual, or bias, field in the vessel being degaussed. This bias field was set to be equal and opposite to that of the vessel, with the result that it effectively cancelled out the distortion the vessel caused as it passed through the earth's magnetic field, making it temporarily indistinguishable from the magnetic background.
Although the system was very successful, it was not a permanent solution, and had to be used correctly. As ships pass through the earth's magnetic field, it tends to magnetise them, and orders were issued instructing captains to make frequent changes to their direction of travel in an effort to minimise this effect. Degaussed vessels had to return to port at regular intervals (between four and six months) to have the process repeated. The polarity of the signal was dependent on their direction of travel, meaning that different magnetic polarities had to be used dependent on whether they were operating in the northern or southern hemisphere.
The original method of degaussing ships was referred to as Coiling, and required required each ship to be equipped with its own set of degaussing coils and supporting equipment to power them and generate the required electromagnetic field. While this method had the advantages of being able to maintain the vessel's magnetic bias continuously, and reverse its polarity for operation in either hemisphere, its complexity and cost meant it was not suitable for installation in all the ships that needed it. This method was used on many British cruisers and battleships, protecting them as they were deployed around the world during the conflict.
Goodeve developed a much simpler and cost-effective for degaussing vessels, known as wiping. Instead of installing coils and control gear in every ship, Goodeve reversed the system and located the equipment at land based Degaussing Stations. Ships would visit these sites periodically, and a cable carrying the degaussing current would be dragged over the hull. The electromagnetic field generated by a high current passing through the cable induced the required magnetic bias into the hull as the cable passed over it, with magnetic hysteresis maintaining the bias thereafter. The cables were substantial, carrying current in the order of 2,000 A. Although there were fears that the constant pounding of the sea, and inherent vibrations in the ship would lead to the bias being dissipated, the main problem was magnetisation of the hull as it passed through the earth's magnetic field and, as noted above, this was minimised by ensuring frequent changes of direction were made whenever possible. The bias on wiped ships gradually weakened over time, and they would be scheduled to visit a Degaussing Station to have their bias restored two or three times each year.
A similar result can be achieved using external coils without dragging the cable over the hull, in which case the process simply referred to as degaussing. In this operation, the coils are either located beneath, or floated around the the ship. Rather than dragging the heavy coils over the hull to distribute the electromagnetic field, a higher initial current (anything from 3,000 A to 4,000 A) is used to generate a larger field. By reducing the magnetising current slowly, and avoiding a rapid collapse of the induced magnetic field, magnetic hysteresis acts to maintains the bias in the hull.
Degaussing inspection stations
The degaussing process was not arbitrary, and had to be carried out accurately to be effective. Applying to much bias, or assigning the wring polarity could have disastrous results. In order to determine the required bias level, ships would visit a Degaussing Inspection Station, where they could be tested, and magnitude and polarity of their initial bias could be measured. Once they had been wiped, they could be retested to determine the effectiveness of the degaussing, and wiped again if the process had not had the required effect. Several passes could be required in some cases.
The inspection station comprised a shore based station which housed the test equipment, and a test loop which would have been laid on the seabed. In operation, the ship being tested would be required to pass over the test loop while the signal generated in the test loop would be detected by the equipment in the station. This would provide a recording of the magnetic signature of the vessel, which would be calibrated and used to determine the required setting of the degaussing equipment.
One such monitoring station is known to have existed at Portkil, on the Rosneath peninsula.
The same degaussing techniques were adopted by the Americans to provide similar protection to their steel hulled vessels, and created their own name to describe the process, which they referred to as deperming, a reference to removing or neutralising the inherent permanent magnetism of steel hulls.
The early wiping system only removed the overall bias of the hull. Later system have become more sophisticated, with monitoring and wiping of the magnetic signature taking place in all three axes to ensure that the bias is effective in all directions.
Development of magnetic mines has also continued, with their sensitivity being greatly increased by having them detect changed in the magnetic field around them, rather than the field itself. This means they can detect small anomalies produced by spots that wiping either misses or cannot reach.
You may add a comment or offer further details which may be included in the page above.
Commenting has been disabled thanks to the attention of scum known as spam commenters
Recent Page Trail: