The fuze is of the floating needle type with a graze pellet carrying the detonator. The needle and detonator are held apart by a centrifugal safety bolt, the out-ward move-ment of which is controlled by a clockwork escapement. The design is similar in principle to the Model 16 described in Pamphlet No. 4, but differs in details.

The overall length of the fuze is 4 inches. When assembled in the shell the 3 inches which protrude consist of a brass tapering body with a red painted aluminium head. The head is tapered near its base to correspond with the shape of the body, but the taper is markedly increased further forward. The nose is flat and is fitted with a brass sealing disc.

The screw-threaded portion of the body for insertion in the shell is 1.76 inches in dia-meter and has a pitch of approximately 3 mm. The body is bored to accommodate the clockwork and graze mechanisms and is closed at the base by two aluminium screwed plugs which are drilled to provide a flash channel. The front end of the body is reduced in diameter and threaded for the assembly of the head. A lateral channel is formed for the safety bolt, and a recess, displaced from the centre, contains a detent and sleeve. A radial channel at right angles to the lateral channel is formed for the locking bolt.

The brass graze pellet carries a 2.2 grain igniferous detonator secured by a perforated brass plug and is designed with a necked portion which acts as a guide for the needle. A steel creep spring is held in compression between a shoulder on the pellet and the base of the brass cylinder containing the clockwork mechanism. The base of the cylinder is bored to fit over the neck of the graze pellet. The detonator filling consists of mercury fulminate 45.2 per cent, potassium chlorate 28.9 per cent and antimony suphide 25.9 per cent.

The brass detent is supported by a three-pronged form of stirrup spring which is attach-ed to its base and rests on the shoulder formed in the brass sleeve. The stem of the de-tent enters a recess in the underside of the safety bolt and retains the bolt in the safe position until "set back" occurs.

The aluminium centrifugal safety bolt is forked at the inner end to proivde two arms which pass under a flange formed on the needle and so prevent the needle moving to-wards the detonator. A recess to engage the detent is formed in the underside of the bolt and a hole is formed in the centre at one side to stem of a spring-loaded centrifugal locking bolt. The opposite side of the safety bolt is in the form of a toothed rack which is emmeshed with the first spur of the clock-work escapement mechanism.

The clockwork arrangement is carried in a brass cylinder which is bored to accommodate the safety bolt and locking bolt. The escapement mechanism consists of a train of four wheels, each consisting of a spur and pinion, and a balance wheel oscillator. The first spur is emmeshed with the rack on the safety bolt. The fourth pinion is an escapement wheel and engages a recess formed in the eccentric projection on the balance wheel occillator. The balance wheel oscillator is contained between the upper and lower motion plates which are suitably spaced by two brass washers. This assembly is secured by two screws to the brass cylinder containing the train of wheels and is surmounted by a spring washer compressed beneath the head of the fuze.

The aluminium head is drilled through the centre to take the stem of the hammer and the aluminium push rod and is recessed at the nose for the hammer head. The recess is clo-sed against air pressure by a brass disc secured by a bush of the same material.

The hammer head has three holes for the escape of the air behind it when the hammer is driven in.

On acceleration the detent sets back, forcing its spring past the shoulder in the sleeve and withdrawing its stem from the recess in the safety bolt. The prongs of the spring then engage the underside of the shoulder and prevent the detent moving forward.

During flight the locking bolt is moving outwards against its spring by centrifugal force and releases the safety bolt. The safety bolt then commences to move outwards, also under the impetus of centrifugal force. The outward movement of the safety bolt is transmitted by the toothed rack on the bolt to the train of wheels, and is controlled by the action of the balance wheel oscillator. The thrown on the projection formed beneath the balance wheel oscillator, under the influence of centrifugal force, performs the nor-mal function of the hair spring in a clock mechanism and, with the recess acting as a pal-let, controls the rotation of the escapment pinion. When the forked portion of the safety bolt has moved clear of the needle and graze pellet, the needle is held off the detonator by the "creep" resulting from deceleration, and the graze pellet is held by the creep spring.

On graze the needle is driven in by the hammer and push rod whilst the graze pellet overcomes the creep spring and carries the detonator on to the needle. The flash passes through the flash channel in the base of the fuze.