BMA III. The BMA III mine was similar in design to the BMA II, except that it was an experimental one-coil mine (mounted on the axis) with nickel specially allocated for the work. The mine was designed to be laid safely from an altitude of 10,000 feet. The coil was wound with copper wire on a Mu-metal core 60 inches long and one inch in diame-ter. The sensitivity of this system was 5 to 10 mg. as opposed to approx. 25 mg. for the BMA II. However, because of a shortage of materials, the development was stopped.