Magnetic units of either the induction or the needle type were feasible; but, since Germany was not self-sufficient in the copper and nickel essential to induction units, work centered upon the needle unit. The first needle type, E-Bik, was completed in 1925, and in subsequent years it was improved and adapted to ground and moored mines. By 1939 the M 1 (unipolar), M 2 (unipolar), and M 3 (bipolar) magnetic units were ready for operational use.

During the war, the Navy continued research for improvements to the M 2, and M 3, and for methods to keep ahead of Allied countermeasures. This resulted, before the end of the war, in the M 4 (unipolar) and M 5 (bipolar). The M 4 was an improvement of the M 3 type, designed for use in either moored or ground mines in combination with other units. It prossessed a maximum sensitivity of 2.5 mg., and contained a device which re-set the unit automatically when it was actuated or disturbed. The M 5, a small, improved M 1 designed for use in ground mines in combination with other units, was abandoned in favor of the more satisfactory M 4. The latter was used operationally; the M 5 was not.

Raw material shortages prevented largescale German production of induction-type units. A limited research program failed to develop substitutes for copper windings and nickel rods. Aluminum windings on high permeability steel rods were tried; but, despite the use of up to eight rods in a single mine case, the loss in sensitivity was too great.

Attempts were also made to develop amplifier induction units for use in combination with other units. The Luftwaffe, which was in a favored position throughout the war, ob-tained small amounts of copper and nickel to develop the AJD 102 and JDA 105 combina-tion induction units. Neither one of these was used operationally.