M 3 Unit Circuit - Operation (Figure 147).

The M 3 (Allied designation M IV and M IVa) bi-polar unit as designed primarily for moored influence mines EMF, LMF, SMA, and TMA.

This unit contains an armature-type, vertically-pivoted needle and represents an im-provement over the M 1 group in that it is more compact, incorporates bi-polar firing, and can be made in mass production. No anti-countermining device is fitted, and the unit fires more readily because of motion. Nevertheless, this unit has been used with consi-derable success in moored and ground mines laid by submarines, surface craft, and air-craft with parachute. An improvement of the M 3 incorporates a special, 15-place me-chanical P.D.M. as an integral part of the unit.

Arming. When the hydrostatic clock runs off its delay period, S1 and S2 close, blow-ing fuse No. 1. This releases a clock escapement and starts A.L.A. S4 makes and breaks during A.L.A., blowing fuse No. 2. Upon completion of A.L.A., S3 closes and the unit is alive.

Normal Firing. A RED or BLUE actuation closes S4 to the proper contact, completing the circuit through the detontor.

Figure 147 – M 3 Unit Circuit

Figure 149 – M 3 Unit – Front and Rear Views

Figure 150 – M 3 Unit for LMF Mine