Fuze
Depending on the technology used, the device may self-destruct[21] (or render itself safe without detonation[22]) some seconds, minutes, hours, days, or even months after being deployed.
By the 19th century devices more recognisable as modern artillery "fuzes" were being made of carefully selected wood and trimmed to burn for a predictable time after firing.
Rifled guns introduced a tight fit between shell and barrel and hence could no longer rely on the flame from the propellant to initiate the timer.
These were soon superseded in 1915 by the Mills bomb, the first modern hand grenade with a relatively safe and reliable time fuze initiated by pulling out a safety pin and releasing an arming handle on throwing.
Impact, percussion or contact fuzes detonate when their forward motion rapidly decreases, typically on physically striking an object such as the target.
A fuze with a graze action will also detonate on change of direction caused by a slight glancing blow on a physical obstruction such as the ground.
Inertial fuzes are triggered when the entity carrying them (for example, a torpedo, air-dropped bomb, sea mine, or booby trap) experiences a sudden (or gradual, depending on the design) acceleration, deceleration, or impact.
Some can be passively safe, ignoring all changes of momentum below a certain threshold, thereby functioning similarly to impact fuzes without the limitation of being externally mounted.
Other designs can be passively dangerous, using other energy sources such as gravity or an electrical battery to greatly amplify slight changes in inertia over time.
Proximity fuzes cause a missile warhead or other munition (e.g. air-dropped bomb, sea mine, or torpedo) to detonate when it comes within a certain pre-set distance of the target, or vice versa.
Proximity fuzes utilize sensors incorporating one or more combinations of the following: radar, active sonar, passive acoustic, infrared, magnetic, photoelectric, seismic or even television cameras.
These may take the form of an anti-handling device designed specifically to kill or severely injure anyone who tampers with the munition in some way e.g. lifting or tilting it.
Regardless of the sensor used, the pre-set triggering distance is calculated such that the explosion will occur sufficiently close to the target that it is either destroyed or severely damaged.
In general, the munition has to travel a certain distance, wait for a period of time (via a clockwork, electronic or chemical delay mechanism), or have some form of arming pin or plug removed.
Detonation of modern naval mines may require simultaneous detection of a series arrangement of acoustic, magnetic, and/or pressure sensors to complicate mine-sweeping efforts.
Alternatively, it can be as complex as the electronic timer-countdown on an influence sea mine, which gives the vessel laying it sufficient time to move out of the blast zone before the magnetic or acoustic sensors are fully activated.
In other cases the bomb, mine or projectile has a fuze that prevents accidental initiation e.g. stopping the rotation of a small propeller (unless a lanyard pulls out a pin) so that the striker-pin cannot hit the detonator even if the weapon is dropped on the ground.
As an additional safety precaution, most modern nuclear weapons utilize a timed two point detonation system such that ONLY a precisely firing of both detonators in sequence will result in the correct conditions to cause a fission reaction [citation needed] Note: some fuzes, e.g. those used in air-dropped bombs and landmines may contain anti-handling devices specifically designed to kill bomb disposal personnel.
Parallel fuze combinations minimize duds by detonating at the earliest activation of individual components, but increase the possibility of premature early function of the munition.
