The number 106 fuze was the first British instantaneous percussion artillery fuze, first tested in action in late 1916 and deployed in volume in early 1917.
Background
Britain entered World War I with a policy of using shrapnel shells for its field guns (13-pounder and 18-pounder), intended to burst above head-height for anti-personnel use. British heavy artillery was expected to attack fortifications, requiring high-explosive shells to penetrate the target to some extent before exploding. Hence British artillery fuzes were optimised for these functions. Experiences of trench warfare on the Western front in 1914–1916 indicated that British artillery was unable to reliably destroy barbed-wire barricades, which required shells to explode instantaneously on contact with the wire or ground surface: British high-explosive shells would penetrate the ground before exploding, rendering them useless for destroying surface targets.
British No. 100 and later Nos. 101, 102 and 103 nose "graze" fuzes available in the field from August 1915 onwards could explode a high-explosive shell very quickly on experiencing a major change in direction or velocity, but were not "instantaneous": there was still some delay in activation, and limited sensitivity: they could not detect contact with a frail object like barbed wire or soft ground surface. Hence they would penetrate objects or ground slightly before detonating, instead of on the ground surface as required for wire cutting. These graze and impact fuzes continued to be used as intended for medium and heavy artillery high-explosive shells.
Up to and including the Battle of the Somme in 1916, British forces relied on shrapnel shells fired by 18-pounder field guns and spherical high-explosive bombs fired by 2-inch "plum-pudding" mortars for cutting barbed-wire defences. The disadvantage of shrapnel for this purpose was that it relied on extreme accuracy on setting the fuze timing to burst the shell close to the ground just in front of the wire: if the shell burst fractionally too short or too long it could not cut the wire, and also the spherical shrapnel balls were not of an optimal shape for cutting strands of wire. While the 2-inch mortar bombs cut wire effectively, their maximum range of limited their usefulness.
Design
Detonation mechanism
The number 106 fuze drew on French technology to provide a mechanism for reliably detonating a high-explosive shell instantaneously when the nose made physical contact with the slightest object like a strand of barbed wire or the ground surface. Hence it was a "direct action" rather than a "graze" fuze: simple deceleration or change of direction would not activate it, only direct physical contact between the hammer projecting from the nose and an external object. The basic mechanism was a steel hammer on the end of a spindle projecting forward from the nose of the fuze. The slightest movement inwards of this spindle caused the fuze to detonate and hence explode the shell before it penetrated the ground.
The steel hammer had a softer aluminium cap which absorbed the force of a glancing blow and prevented the spindle from bending or breaking, reducing the risk of misfire.
Arming and safety features
The first safety mechanism was a length of brass tape wrapped around the spindle between the fuze body and the hammer head, which prevented the spindle from moving inward. On firing, the hammer's inertia caused it to "setback" fractionally i.e. it resisted acceleration, and hence the accelerating fuze body forced the tape wrapped around the spindle against the underside of the hammer head, preventing the tape from unwinding. When acceleration ceased shortly after the shell left the gun barrel, the hammer and fuze body were travelling at the same speed and the hammer ceased to "set back", freeing the tape. The shell's rotation then caused a weight on the end of the tape to unwind the tape through centrifugal force, hence activating the fuze. Because of this, usage of this fuze in action was characterised by British troops in the front lines noticing the descending tapes detached from the fuzes as they travelled overhead towards the enemy lines.
After the tape detached during flight, the hammer was prevented from being forced inward by air resistance by a thin "shearing wire" passing through the hammer spindle, which was easily broken by the hammer encountering any physical resistance. The spindle was prevented from rotating relative to the fuze body in flight, and hence from snapping the shearing wire, by a guide pin passing through a cutout in the spindle.