M 4 Used in Combination with Another Unit.  When the M 4 is used as a straight magnetic unit, it may permature, because of faulty settings or variations in the earth's magnetic field. On the other hand, when used in combination with others units, the M 4 will reset itself when actuated to compensate for the magnetic influence exerted. The mechanism designed to accomplished this resetting is called ZR IIb, and is used only when M 4 is combined with AT 2 or AT 3 (these combinations are designated AMT 2 and AMT 3).

The circuit of M 4 with ZR IIb is shown in figure 155. The switch cam of ZR IIb is ac-tually fitted with six contacts, but for simplicity, only four are indicated. When the bat-tery is connected into the circuit, relay A is energized through contact T. When T opens, A remains energized through BLUE contact K1. Current passing through A causes a1, a2, and a3 to operate. Contact a3 maintains self-holding of A, a2 starts motor M, and a1 switches over. At this point, it is necessary for contact II to be closed in order for lati-tude adjustment to take place. Thus, approximately 30 seconds of each two-minute cycle is available for magentic adjustment. With the above exeptions, latitude adjust-ment occurs in the normal manner.

Figure 155 – M 4 Unit Circuit (with ZR Ib)

When latitude adjustment is complete, the unit is ready for actuation. If a RED mag-netic actuation occurs, K2 is made and relay B energized through III and H2. When B is energized, b1, b2 and b3 operate. Contact b2 maintains self-holding of B; b2 starts the cam-switching motor, and b1 switches over, but does not energize M2, since II is open. A few seconds later, I closes and maintains motor current until the end of the cycle. Thereafter, IV closes, energizing the associated combination units. If the mine does not fire, IV reopens after a set interval and II closes. If, at this point a different magnetic field prevails, the motor M2 read-justs the M 4 unit accordinbly. If the unit is still proper-ly adjusted, A is energized, and the adjusting current through R and M2 is broken. if a new adjustment has been necessary, the same series of events will occur after the ad-justment. At a point near the end of the switching cycle, II and III open. The opening of II limits the period during which readjustment may take place. The opening of II breaks the holding circuit to A and B, after which III recloses. When I closes, the switching cycle ceases. If the closure period of II is insufficient to permit proper readjustment, the needle switch remains on contact and a new cycle is started. This process continues until proper adjustment has take place.