The unit and P.D.M. are rendered active when the hydrostatic clock switches closes, figure 296. A magnetic actuation closes the needle switch, and the air core relay closes S5 to contact No. 1 and energizes the P.D.M. solenoid through contacts R-S. The solen-oid releases wheel G and the P.D.M. clockwork.

1. In the twelve-place P.D.M., G rotates one revolution, while A rotates 1/12th of a revolution clockwise.

2. In the six-place P.D.M., G rotates one revolution, while A rotates 1/6th of a revo-lution clockwise.

After 11 seconds of rotation, G allows P-Q to break, de-energizing the unit hold-on coil and the P.D.M. solenoid. After 120 (45) seconds. P-Q- remakes and the wheel stops.

A movable pin with a flanged collar may be put in any setting hole, and is indicated as being in hole No. 6 in the accompanying photograph. In the case illustrated, five actu-ations of the P.D.M. would cause the flanged collar to bear against the U-bend in the spring contact S, causing it to break contact with R and make with T. This substitutes the detonator for the P.D.M. solenoid in the circuit.

The six-place P.D.M. is used with M 1 units and has a 45-seond operating period per "blind" actuation. The twelve-place P.D.M. is used with A 1 and MA 1 units and has a 120-second operating period per "blind" actuation.

Figure 296– Mechanical P.D.M. and Circuit