Basically three mechanisms have been used: a. Powder burning, these were the norm with shrapnel shells in WW1, and continued with smoke shells until at most a

Basically three mechanisms have been used: a. Powder burning, these were the norm with shrapnel shells in WW1, and continued with smoke shells until at most a few decades ago. b. Mechanical time, appeared in WW1 and still used for shells that need a high burst such as smoke and illuminating. Were also used for HE, but round to round variation meant they were sell that perfect. c. Proximity (AKA Variable Time (VT)), this uses a radio transmitter and relies on reflected radio waves, first used in WW2 for anti-aircraft. Based on UK development that was transferred to the US in 1942. Used in the ground role to get a burst about 10 metres above ground late in WW2. Now a function in Multi-role fuzes that provide airburst, ground burst or delayed burst (ie penetrate the ground for about a metre before bursting) as selected.