Page:EB1922 - Volume 30.djvu/162

128

4 oz. 13 dr. Size 2>/ 4

Leather Board Cup

...401. 5 dr. Size 4'/,

Primer FIG. I. 4-5 in. Q. F. Howitzer Cartridge.

oxalate and mealed powder enclosed in a shalloon bag and placed between the projectile and the cartridge was tried during the war. The addition of mineral salts such as sodium or potassium chloride has also been tried; but so far the results have not been very satisfactory.

Means of Ignition. Amongst the various methods that have been evolved for the firing of the gun only the friction tube, the percus- sion cap, and the electric bridge are now in general use, and of these the friction tube is practically confined to old models of guns con- tinued in the service of various countries owing to the need of all available material in the early and middle stages of the World War.

With the percussion tube 1 ignition of the powder in the body of the tube is obtained from a blow on the head of the " striker," which drives a percussion cap against a hollow brass anvil. The cap consists of a copper shell, cup shaped, coated on the interior with fine varnish; this shell is Tilled with a chlorate mixture, a thin tin- foil disc is pressed in, and a coating of varnish applied in order to prevent excess of moisture. Internal sealing is obtained by the shell of the cap being expanded into its seating by the force of the explosion. 2

There are two types of electric tube, one with external wires for joining up with the electric circuit and the other without external wires. In the former two insulated wires are led into the interior, and in each circuit with these there is a wire " bridge " of platinum silver surrounded by a priming composition of gun-cotton dust and mealed powder. On an electric current passing, the bridge is heated to incandescence and ignites the priming composition. In the second type (see fig. 2) the breech mechanism of the gun makes electric contact with an insulated disc in the head of the tube; this disc is connected by an insulated wire to an insulated brass cone, the bridge being formed from the edge of the cone to a brass wire soldered to the mouth of the tube; priming composition sur- rounds the bridge. The electric current passes from the breech mechanism to the disc in the head of the tube, thence through the bridge to the body of the tube and through the metal of the gun. Internal sealing is obtained by the cones being driven backwards into conical seatings.

BRASS FULL SIZE

Paper disci and

Cork plug

Loose powder .... Copper pole

Glazed board disc.and Paper disc attached

Iridio-Platinum wire Composition priming Ebonite

Silk covered wire

Ebonite Contact piece itiru

FIG. 2. Tube, Vent Sealing, Electric W. P.

1 In Great Britain the term " tube " is officially used to designate the smaller firing devices of this class which are pushed into posi- tion, and " primer " for the larger ones which are solidly screwed into the base of Q.F. cartridge cases. In the United States the term " primer " is common to both. (C. F. A.)

2 In some instances during the war period firing was done by means of a cut-down service rifle screwed into the breach of a gun; in this case an ordinary blank rifle cartridge acted as a percussion tube. (C. F. A.)

With Q.F. cartridges the means of ignition are carried in the base of the brass case, and may be either (a) a percussion cap, (b) a percussion or an electric primer, (c) a percussion or an electric tube held in an adapter. The percussion cap is precisely similar in principle to that of a rifle cartridge and needs no description. The primer is used with larger guns and affords easily replaceable means of ignition. In both percussion and electric types the body of the primer is made of an alloy resembling brass; externally it is screw-threaded to screw into the recess prepared in the base of the cartridge case, internally it is recessed to form a magazine. The percussion primer is fitted with a percussion cap resting on an anvil pierced with flash holes; the anvil is recessed to hold a copper ball and retained by a screwed plug also pierced with flash holes. The action is the same as with a percussion tube; internal sealing is obtained by the copper ball being driven backwards in the coned recess in the anvil. The electric primer is similar to the vent-sealing electric tube in construction and action.

The primer is being superseded by a vent-sealing tube held in an adapter externally of the shape of the primer. The adapter is boced internally to receive the vent-sealing tube, percussion or electric, which is retained in position by a small stud operated by a spring. Attached to the front of the adapter is a metal container filled with a small charge of gunpowder to augment the flash from the tube. (F. M. R.)

FUZES

A fuze is the device or mechanism that ignites the bursting charge of a shell fired from a gun, howitzer or mortar. Fuzes fall into two categories, those which burst or open the projectile in flight (time fuzes), and those which burst it on impact or graze (percussion fuzes). Of the former all, with the exception of the recently introduced clockwork fuzes, rely for their action on the known speed of burning of a readily ignited composition. In the days of muzzle-loading guns the flash of the powder charge ignited this composition, but in the modern breech-loading guns the passage of the burning gases is checked by the driving band of the projectile, and other means have to be employed for its ignition.

The percussion fuzes in nearly all cases rely for their action on a movable pellet in the interior which held in position by a shearing wire, centrifugal bolts, the direct pressure of the powder gases (as in some base fuzes) or other means is released by the shock of discharge and is free to move. The fuze is then described as " armed." 3 The pellet is provided with a disc of detonating composition at the end which is foremost when the shell is in the gun and on graze or impact the pellet flies forward, and the patch of detonating composition impinges on a sharp point or " needle " in the front end of the fuze, the flash igniting a charge of gunpowder or other explosive in the " magazine " and this in turn igniting the bursting or opening charge of the shell.

In the large proportion of time fuzes the same principle, i.e. the movable pellet and detonating patch, is relied on for the ignition of the ring or rings of composition. A precaution is necessary, however, with regard to these pellets when free to move in the interior of the fuze, as it has been found that they, not being exposed to air resistance, have a tendency to move forward as the shell loses velocity, and thus to cause premature bursts in flight. To counteract this tendency weak spiral or " creep "-springs are so fitted as to control the forward move- ment of the pellet. There are other additional devices to secure the proper arming and subsequent action of the fuze which will be described in due course.

All fuzes are screwed into a bush or adapter either in the head of the shell (nose fuzes) or, in case of solid-pointed shell, into the base (base fuzes). With certain " false-cap " shell the fuze is internal, that is, inside the false caps, but it is in effect a nose fuze in that it is placed in the front of the explosive container.

Percussion Fuzes. Among percussion fuzes the simplest are those known as direct action, and a British example known as Fuze No. 44 is shown in fig. I. This fuze is provided with a safety shutter a device to which reference will frequently be made in the sequel and for safety in transport is fitted with a cap and with a safety-pin which blocks the moving parts. On loading, the cap and safety-pin

8 All fuzes before acceptance as service stores are subjected to rough-usage trials to test their powers of resistance to shocks during transport, and it will be understood that the process of " arming " is necessary both to secure this and to prevent premature action in the gun.