Proximity or VT Variable Time Artillery Fuzes

These fuzes, sometimes called Posit Buck Rogers Special Influence and Bonzo are in effect automatic time fuzes . Without setting or adjustment they detonate the missiles that carry them on approach to the target at the desired point on their trajectories, such as 

When this interaction of transmitted and reflected waves (resulting in ripples or beats), reaches a predetermined intensity, it trips an electronic switch, which then permits an electric charge stored in the firing capacitor (condenser) to flow thru an electric firing squib. 

The VT fuzes can be used only in deep-cavity projectiles (booster cavity is 2.75 inches deeper than for PD, MT and TSQ fuze booster cavities) without the supplementary bursting charge 

It seems that VT fuzes for bombs were invented by the Germans before WWII and some of them are described below under Bomb Fuzes . British also started to work on VT fuzes before WWII and then, in August 1940 work started in US. By 1943, the US Navy started to use them against Japanese planes, especially the Kamikadze (suicide) planes. In 1944 US Army used them successfully at the battle of Ardennes 

A supporting device (I) prevents the vial from moving to the rear until a setback force of some deration acting upon the glass vial forces the leaves of the device to open. 

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Artillery fuzes, proximity or VT (variable time) 4D918... 

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