The M 4 Unit is a bi-polar, dip-needle, magnetic unit. It is of the same basic type as the M 3, but it is smaller and more compact. It is capable of dependable setting at a 2.5 mg sensitivity, and was used operationally in 1945.

The M 4 may be used alone in moored or ground mines, or as a magnetic trigger in a combination unit. In the former case, the sensitivity is adjusted to 20 mg; in the latter case, sensitivity settings may be as high as 2.5 mg. When used in combination, the M 4 is capable of repeated resetting to compensate for magnetic variations or faulty original adjustments.

The major differences between the M 4 and M 3 units are as follows:

1. The M 4 is capable of higher sensitivity setting.

2. The M 4 has a more dependable latitude adjustment system.

3. The orientation of the M 4 need not be vertical.

4. The M 4 is capable for re-setting when used in combination.

5. The M 4 is much smaller, or more rugged construction, with fewer moving parts.

The M 4 was developed to meet the need for a bi-polar unit of high sensitivity which would not fire prematurely as a result of mine motion. When used as a straight magnetic unit, the M 4 is mounted in gimbals for vertcal orientation.

The use of M 4 on a fixed-axial orientation in the LM or TM type mines in combination with AT 2 and AT 3 was in development. These combination were designated AMT 2 and AMT 3.

Figure 151 – M 4 Unit

Figure 153 – M 4 Unit in Housing