those relying on direct impact with the object. The British base percussion fuze illustrates
type (1). In this, before firing, the needle pellet is held
back by a central spindle with a pressure plate attached to
its rear end. For additional safety a centrifugal bolt is
added which is released by the rotation of the shell. On
discharge, the gas pressure pushes the pressure plate in, the
central spindle is carried forward with it and unlocks the
centrifugal bolt; this is withdrawn by the rotation of the
shell, and the needle pellet is then free to move forward
and explode the detonating cap when the shell strikes.
Type (2) is that usually adopted in small base fuzes and in
the percussion part of "time and percussion" fuzes. Here the
ferrule, on shock of discharge, moves back relatively to the
percussion pellet by collapsing the stirrup spring; this leaves
the pellet free to move forward, on the shell striking, and
its detonator to strike the needle fixed in the fuze body. A
spiral spring prevents any movement of the pellet during flight.
The direct-action or impact fuzes of type (3) are very simple. They are made of such
a strength that during discharge nothing happens, but on striking an object the needle disc is crushed in and the needle explodes the detonating composition and thence the powder.
The action of all time fuzes is started by the discharge
of the gun. By this the pellet strikes the detonator and
so ignites a length of slow-burning composition which is
pressed into a wood tube or into a channel formed in a metal
ring. To regulate the time of burning of the wood fuze, a
hole is bored through into the composition as before stated,
so that when it has burnt down to this hole one of the side
channels filled with powder is ignited and explodes the
shell. Wood fuzes are now only used for R.M.L. guns.
With modern long-burning fuzes, two composition
time rings are used. The lower of these rings is made movable
so that it can be turned to bring any desired place over a
hole in the body of the fuze, which is filled with powder and
communicates with the magazine. On the gun being fired the
detonator is exploded and its flash ignites the upper time
ring. This burns round to a passage made in the lower ring, when
the lower ring begins to burn and continues to do so until the
channel to the magazine is reached. The gases from the ignited
composition escape from an external hole made in each time ring.
Mechanical time fuzes depending on the rotation of the shell
to give a regular motion to clockwork have been tried, but
so far no practicable form of these fuzes has been found.
It is important that all fuzes should be rigidly guarded against
dampness, which tends to lengthen their time of burning; hence
they are protected either by being kept in hermetically sealed
tins holding one or more fuzes, or by some similar means.
References
