ELECTRICAL PRINCIPLES. To illustrate the principle of the three circuit electrical fuze, a schematic diagram is shown in figures 123 and 130.

A. Charging Circuit Operation. The charging circuit in the plane is of special interest. As the bomb is loaded in the aircraft, the charging head is clamped on the fuze head. The charging pins contact the plungers and depress them to that they make electrical contact with the storage condensers. The two charging pins are connected to the sliding contacts located in the charging arm. These contacts close when the bomb has fallen from one to three inches from the rack. This prevents charging of the fuze while the bomb is still in the aircraft. The two sliding contacts are connected to the positive termi-nal of the 240 V battery. The B plunger circuit is connected directly, while the A plunger circuit is connected through a "ZSK" or selector switch. This switch has two positions: open (MV), with delay; and closed (OV), without delay. The battery is tapped at 240 volts and 150 volts. These two leads run to the voltage switch. The voltage switch can be set at 150 volts for level bombing or 240 volts for dive bombing. It cannot be used to open the circuit. The voltage switch is connected to the master switch which is used to jettison the bombs. Supposedly it is closed only when the aircraft is over enemy terri-tory. The master switch is connected to the charging head which contacts the fuze head and completes the electrical circuit throughout the fuze body to the storage con-densers.

Figure 123 – Wiring Diagram of Three Circuit Electrical Fuze

B. Fuze Circuit Operation. For the purpose of illustration we shall assign arming times for the three circuit as follows: Instantaneous circuit, 8 seconds; short delay circuit, 8 seconds; long delay circuit, 2 seconds.

When the selector switch is closed, both storage condensers are charged in the manner described in the above paragraph. (See fig. 123.) The charge placed on the storage con-denser C-1 leaks slowly through the resistance R-1 into the firing condenser C-2. The time required for the curent to pass to the firing condenser C-2 and build up sufficiently to fire the igniter is called the arming time. At the same time the charge on the storage condenser C-3 is leaking through the resistance R-3 into the firing condenser C-4 and through the resistance R-2 into the firing condenser C-5. On impact the trembler swit-ches, S-1, S-2, and S-3 mo-mentarily make contact with the trembler cavities causing the current to flow through the igniter bridges. The bridges become heated and fire the match composition surrounding them. When all three igniter bridges fire at the same time the instantaneous bridge fires the flash pellet and detonates the bomb trough the normal explosive train. The short and long delay trains just started to burn at the time of deto-nation.

If the selector switch is held open, then no charge will pass to the instantaneous circuit but the B plunger, remote from the locating pin, will carry the charge to the storage con-denser C-3. The circuit through the resistance R-2 to the condenser C-4 becomes armed before the circuit through R-3 to C-5. If the bomb has been dropped with a falling time of less than 8 seconds, the latter circuit will not be armed before impact and the igniter bridge used with the trembler switch S-2 will fire the long delay pellet which, acting through the explosive train of the fuze, will detonate the bomb. If the bomb is dropped with a falling time or greater than 8 seconds, both circuit will be armed before impact but because of the shorter delay train used in conjunction with the trembler switch S-4, the short delay will initiate the final explosive train.

Time fuzes contain essentially the same basic parts as the impact fuzes except that the trembler switches are replaced by a vacuum tube which becomes conducting at a critical known voltage. At the instant the bomb is started on its trajectory, an electric charge is placed on the storage condenser, and another smaller charge is placed on the firing con-denser. The time setting of the fuze is adjusted by varying the amount of charge placed on the firing condenser. During flight, part of the charge on the storage condenser leaks through the resistor to the firing condenser. As the charge on the firing condenser in-creases, the voltage across the tube increases. When the firing voltage of the tube is reached, the firing condenser discharges through the tube and igniter, firing the fuze.

Both the storage and the firing condensers of German electrical time fuzes are charged in order that the voltages used can be kept reasonably low. An increased time setting re-quires an increased voltage.

Figure 129 – Electrical Symbols