When assembled, the reserve energizer is inert and does not supply energy to the elec-tric circuits of the fuze. The reed switch is closed, thus placing a short circuit across the firing condenser. The mercury switches are closed, placing one or two short circuit across the firing squib. The auxiliary detonating fuze is also in the unarmed condition. The fuze remains in this condition during transportation and storage and until the projec-tile in which it is assembled is fired from a gun.

When the projectile is fired from a gun, the ampoule in the reserve energizer is shattered on the breaker as a result of set-back forces, and the electrolyte is freed. Centrifugal force distributes the electrolyte to the battery cells, and the reserve energizer becomes active. Centrifugal force acting on the reed of the reed switch causes it to open imme-diately upon firing, thus removing the short circuit it had imposed across the firing con-denser. Centrifugal force acting on the reed of the reed switch causes it to open imme-diately upon firing, thus removing the short circuit it had imposed across the firing con-denser. Centrifugal force causes the mercury in the mercury switch to be forced from its inner chamber through the sintered metal cup into the outer chamber of the switch, thus removing the electrical short circuit it had imposed across the squib. A short predeter-mined time is required for this action.

The auxiliary detonating fuze is also armed by centrifugal force. Centrifugal force throws the rotor detens outward, freeing the rotors. After set-back forces are expended, centri-fugal force causes the rotor to rotate into the armed position, so that their explosive charges are in line with each other.

After the energizer is activated, it becomes a source of power for the electrical part of the fuze; the firing condenser begins to receive a charge, and the transmitter starts to radiate a radio signal.

When the firing condenser is charged, the mercury switches open, and the auxiliary de-tonating fuze is armed, the fuze is fully armed. The elapsed time varies from 0.3 seconds to 0.9 seconds in projectile fuzes and from 4 seconds to 12 seconds in spin-stabilized rocket fuzes.

The radio signals that are radiated by the fuze are reflected from such targets as metal objects, water, earth, etc. The engine and control cables of wooden airplanes will also reflect signals. When the projectile comes into close proximity to the target, the reflec-ted signal from the target reaches the required amplitude and allows the thyratron to fire, thereby discharging the firing condenser through the squib. The blast from the squib operates the auxiliary detonating fuze, which in turn initiates the detonaton of the pro-jectile.

In early model VT fuzes with dry energizers, now obsolete was somewhat different from present wet-energizer fuzes. Instead of the mercury switches, the fuzes contained a mechanical switches, the fuzes contained a mechanical clockwork similar to Time Fuze Mk 18, which ran for 0.4 to 0.6 second after leaving the gun, at which time it removed a mechanical gate from the canal between the auxiliary detonating fuze and the electrical squib. An electrical short circuit across the firing squib was broken only by operation of the clockwork. Set-back force, incident to firing, closed set-back switches which con-nected the dry energizer, which was the source of power, to the electrical circuit. Ope-ration after arming was the same as that in the wet-energizer fuzes.

In the VT fuze for spin-stabilized rockets, setback force is not depended upon to break the electrolyte ampoule, because the force is far less in a rocket than in a projectile fired from a gun. The ampoule is brocken by means of a spin breaker, descriped in Chapter 2.