Rheinmetall Bomb Fuze - Type 157/3. While the mine is being carried by the laying air-craft, the two spring-loaded charging plungers are depressed. As the mine is released and starts to drop, a potential of 180 volts is applied to the plungers, which act as a po-sitive terminal, the fuze body being the negative terminal. When the mine clears the plane, the plungers spring up and arm the fuze firing circuit.

The fuze has two functions, as follows:

1. If the mine strikes a hard surface producing a deceleration of over 200g, an inertia bolt switch closes, firing the instantaneous detonator.

2. If the mine strikes a surface producing a decelaration of between 20 and 200g, one or both of two vibrating "trembler" switches closes, discharging the condenser through an electric igniter in the master switch.

Figure 93 – Diaphragm Microphone

Figure 94 – Rheinmetall Fuze Circuit

Master Switch. The master switch is a positive-locking, single-pole, single-throw type, the body of which contains two spring-loaded contact plungers which bear against a contact block inside the firing device. Each of these plungers in turn is connected to an additional spring-loaded contact plunger which bears against an insulated portion of the contact bridge plunger.

When the Rheinmetall fuze discharges through one or both of the switch igniters, a thermit cartridge ignites. This in turn ignites combustible material inside the switch, and the resulting heat melts a plastic plug which holds the contact bridge open. Spring tens-ion then forces the contact bridge plunger into the molten plug, thereby bridging the two side contact plungers and closing a switch in the firing circuit. A spring-loaded detent holds the contact bridge plunger in the closed position.

Fuse Delay Switch (Figure 96). This switch is an electrically-operated delay type used in conjunction with delay bomb firing and period delay mechanism operation in this mine. It consists of a small cylindrical shell, D, mounted in a fuze clip, L, on an insulating board, I. A circuit is made from H to K through the spring copper strip, G, down the spring-loaded spindle, F, which is held in place by the adhesive action of a soft solder plug, B. The circuit continues from the spindle into the heater coil, A, and out through the shell, D. At the end of a heating period predetermined by the basic switch design, the solder melts and spring, E, forces spindle, F, to the left, thus breaking the circuit from H to K and making a circuit from H to J through the conducting strip, G.

Figure 96 – Fuse Delay Switch