The Detonator Head
��Although it explodes a giant shell it is harm-
PERCUSSION PELLET PERCUSSION SPRING
��THOSE great shell cra- ters which form such commodious quarters for the Tommies to stretch in after weeks of confinement in narrow trenches or which may be improvised into forts on occasion, are mostly formed by the explo- sion of high-explosive shells fitted with de- tonator heads, al- though similar craters may also be formed by detonated shrap- nel. The shattering of fortifications, the demolition of supply depots and most of the damage caused by explosive shells is made possible through the agency of the small detonator fuse or head fitted to the nose of each projectile.
This little device is simple in construc- tion and is of interest in that two distinctly different forces are required before it will become effective — a centrifugal force for unlocking the mechanism and a detonating force for the explosion.
On being fired from the gun, the projectile travels at a high rotary speed and the dedent spring is compressed, so that the dedents, acting as virtually one piece, drop down into the dedent spring cavity. The twirling of the shell throws the upper de- dent out of plumb as soon as it drops into the spring cavity. On the recoil of the spring the dedents lock into the larger hole. The twirling of the shell, due to the rifling of the gun, also causes the centrifugal bolt to fly away from the graze pellet once the upper dedent has dropped into the lower cavity, so that the graze pellet is held in position simply by the coiled spring between it and the cap. In the top of the graze pellet is fitted a detonator charged with a highly explosive material. On the arrest of the flight of the projectile, through
���less until it leaves the muzzle of a gun
��PERCUSSION NEEDLE PERCUSSION DETONATOR
��The body of the detonator head carries within it a cylindrical block of material known as a graze pellet, while the cap of the head carries the percussion needle. An alternate detonating device is also provided in the body — one which becomes effective only after the graze pellet has been released by contact. This second detonating mechanism is provided simply to assure explosion and is probably seldom essential
��contact with a solid body, the graze pellet with its de- tonator is thrown violently against the needle in the cap and exploded. The re- sulting flash passes through a central hole in the graze pellet and ignites the powder in the gaine tube and is transmitted through it to the explosive charge at the base of the shell.
Protruding from the lower end of the graze pellet is a teat which fits into a hole in a kind of transverse carriage known as the percussion pellet. This pellet carries at one end a holder containing at its center the percussion needle around which are a number of flash holes giving direct com- munication through the percussion pellet to the top of the gaine tube. Opposite the percussion needles carried in the body of the head is a second charged detonator. Behind the percussion pellet, or, rather, within the hole bored in its opposite end, is a spring, held in compression by the teat of the graze pellet. When the graze pellet is thrown forward by the contact of the projectile, this teat is withdrawn from the hole in the percussion pellet, releasing the percussion spring. This throws the per- cussion pellet forward forcibly, resulting in the contact of the percussion needle with the second detonator. The flash created is transmitted through the flash holes surrounding the needle and passes to the top of the gaine tube, thus doubly assuring ig- nition of the explosive charge in the shell. This second detonating arrangement can not operate until the graze pellet has been thrown forward into contact with the graze needle, and the graze pellet cannot travel forward until the centrifugal bolt has been thrown out of place, which, in