M 2 Unit Circuit - Operation (Figure 144).

The M 2 (SM-BIK) unit was a dip-needle type unit designed for and laid in the TMA mine. It was considered unsuccessful and experimental, and was withdrawn from service.

Arming. When the hydrostatic clock runs off its delay setting, a-g and b-c close, blowing the detonator release fuse and startin A.L.A. which is performed as in M 1. The M 2 differs from M 1 as follows:

1. The firing current passes directly through the needle switch.

2. The anti-countermining device is of a different type.

3. The needle switch consists of eight flat needles mounted on a horizontal shaft with the needle edges in the vertical plane. The shaft is reverse-geared to a counter-balance system of weights and lever arms, so that the moments of inertia of the needles and counterbalance system are equal and opposite. The above change in construction per-mits the unit to tilt as much as 45° from the vertical without closing the needle switch. The net result of this modification, then is that the opposing inertia moments prevent the unit from firing from shock or tilting, whereas the needles may depress and close the needle switch upon receipt of a firing actuation without affecting the counterbalance system.

Normal Firing. A firing actuation closes the needle switch, operation the hold-on coil and completing the circuit through the detonator.

Normal Anti-Countermining. A countermining shock opens the inertia-operated clock-work-type anti-countermining switch, which is normally closed. When the switch opens, it winds a small clockwork escapement, and the cirucit is broken until the escapement runs down in 3 to 10 seconds, varying directly with the intensity of the shock.

Figure 144 – M 2 Unit Circuit

Figure 145 – M 2 Unit Breakdown

Figure 146 – M 2 Unit In TMA Mine