Methods of Arming.
Spring method.—Let us suppose that after our projectile has started on its way the sliding block is free to move within a cavity at the forward end of which is the anvil. If the projectile comes to a sudden drop or even sudden reduction of velocity the block if unrestrained will, according to the principle of inertia, keep on going till something stops it. The something in this case is the anvil and the fulminate cap is set off. But it is not so simple. For while the projectile is in flight it is acted upon by the air resistance and slows down but the block in the cavity of the head is not subjected to this resistance. It therefore gains on the projectile or creeps forward in the cavity unless restrained as it is by a spring. Now one more point and this type of fuze is complete. We supposed that our block was free to slide. For safety’s sake it is pinned to the cavity. Again we call upon inertia to bread the pin so as to leave the block free to slide. The strength of the pin is calculated so that the force of inertia of the mass of the block is greater than the resistance of the safety pin and when the projectile starts the pin breaks and the spring forces the block to the rear of the cavity until the sudden stop of the projectile permits the block to slide forward as explained. Such a fuze requires a comparatively high initial velocity and is not adapted to howitzers using low muzzle velocities.
There are three other methods in use to arm the fuze. They are inertia of a sleeve; centrifugal force and powder pellet system, that is, combustion of a grain of powder holding the sliding block from the anvil by means of an arm resting against the unburned powder grain. These are more sensitive than the type described.
In the first system, a sleeve fitting around the plunger carrying the cap slides to the rear by inertia when the projectile starts and two clips engage in notches on the plunger body making the sleeve and plunger thereafter move as one body, they are thus held together by a plunger spring which before arming held the plunger away from the anvil. The safety spring held the sleeve and plunger away from the anvil and after arming prevents forward creeping by the plunger and sleeve now locked together. Upon striking, the plunger and sleeve move forward as one body and the cap strikes the anvil.
In centrifugal systems the primer plunger is kept safely away from the anvil by a lock which is kept in place by springs. When the rotational velocity reaches a certain point the force of the springs is overcome by the centrifugal force and the locks are thrown aside or opened and the plunger is free to move forward on impact.
In the powder pellet system (the one largely used by the Germans) there is a well or channel filled with compressed powder, this is set off by a fulminate cap which is fired by inertia, a small plunger-anvil striking the cap. When the powder is consumed it leaves a channel into which an arm attached to the sliding block carrying the igniting fulminate for the charge may slide, thus permitting the block to slide forward to the anvil fixed in the forward part of the cavity. It is held from creeping forward after the compressed powder is burned by a safety spring, thus insuring sufficiently hard an impact to set off the cap.
Heretofore in our service the fulminating cap has been fixed and the plunger carried the anvil or as we call it the firing pin. Such is now the system in our base detonating fuzes, and in our combination fuze.
The new point detonating fuzes are patterned after the French and are practically French fuzes.
Fuzes Classification.
Fuses are classified as: