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The paper describes a series of tests of the disturbance caused in a uniform alternating magnetic field by an incoming magnetic mass looking to the development of a device for the detection of enemy submarines. Detection tests in the laboratory with submarine models 10 ft. long are in agreement with underlying theory and may be used as a basis for the design of larger equipment. Detection tests on steel hulls from shore equipment and from equipment installed on a wooden submarine chaser 110 ft. long are in good agreement with results predicted from a formula developed in the laboratory tests. By sufficient increases in the size of the equipment it is possible, in quiet water, to detect a steel vessel 200 ft. long at a distance of 500 ft. The weight of equipment required would be about 8000 lb. Distances of detection in the neighborhood of 200 ft. were obtained in quiet water with equipment weighing 4500 lb. installed on a wooden submarine chaser 110 ft. long. The reliability of the system of detection is immediately impaired by motion of the vessel, due to her engines, heavy sea, and changes of course, or speed. The results mentioned involve the use of amplifier tubes for enlarging the signal. Without the amplifier the distances of detection are very short, even when a large magnet or large detecting coil and an extremely sensitive detecting instrument are used. With amplifier, detecting coil and detecting instrument higher sensitivities may be reached than can be used in the detection tests. Disturbances in the detecting system fix the limit for increasing sensitivity. These disturbances are amplified and appear in the detecting instrument as larger than the signal sought.