Answer:
The main idea behind MADs is that they use the principle that metallic objects (submarines) interfere in the magnetic lines of Earth forces. And this equipment detects this anomaly. Submarines, among many metallic things, can be detected by using MADs.
The history of MAD development
MADs are Magnetic Anomaly Detectors. Their function is to detect anomalies in the Earth’s magnetic field caused by submarines.
Submarines are made of ferrous metal which contains domains that are tiny magnets formed of iron molecules. This material creates a disturbance that can be detected in the magnetic field.
In 1917 the interest in the detection of submarines started with the study of hydrophones. Then in 1918, the U.S. considered using magnetism in this area but it did not result to be practical as it had a limited detection range. Then by 1941, Britain and the U.S. developed magnetic detection devices to measure Earth’s magnetic field, so after that in 1943, MADs were installed in ASW aircrafts. Due to the limited range and its lack of ability to detect the magnetic variance from different sources, MAD started to be used in combination with sonobuoys (a portmanteau of sonar and buoy). This combination allowed an aircraft to detect submarines with the confirmation of sonobuoys. To reduce the interference from electrical equipment or metal in the aircraft, the MAD sensor is placed at the end of an aerodynamic device. Even so, a submarine must be very near the aircraft's position and close to the sea surface for detection of the anomaly, because magnetic fields decrease over distance.
Besides the use of finding submarines, MADs are used by geologists and geophysicists for determining the distribution and concentration of magnetic minerals and mineral deposits.
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