The original poster was talking about 20mm "Proximity Fuses" Another was talking about them being magnetic. They were in fact small radars inside the fuse, and they were only produced in the 5" range then finally in the 4" range (90mm)

Prox or VT fuses were not authorized for use in the ETO until December 1944/January 1945. That is LATE WAR :) 5" Naval guns are the biggest AAA used in the Pacific by the allies.. they are THE "big guns" for Prox Fuses. They were allowed to use them earlier at sea for AAA since the odds of capture were almost zero. In the ETO 155 VT Fuses were devastating as they could be reliably airburst over massed infantry with incredible results.

There is a very good chow in History channel on its development.. the G Forces involved made the reliability quite difficult to acheive.. amazing they were able to actually..

I have some German wartime cannon rounds and found that rounds had a very simple self destruct mechanism.
The tracer component at the base of the cannon projectile and the main explosive charge in the front of the bullet were connected by a small hole.
The tracer element would burn through the hole and explode the shell.

Rgr that. Being mostly PTO fan, I'm not familiar with the history of their usage in ETO :D. Will read some more about it, thanks!

The reason they overruled the order not to use them was because of the Battle of the Bulge... It was considered desperate enough to make full use of them. German VG Infantry units got literally shredded by massed Time on Target 155 fire using the VT fuses.. Typically with Arty most of the casualties are caused in the first 10-12 seconds before the "target" has a chance to seek cover.. using TOT techniques all the rounds arrive over the target at the same time.. using VT fuses meant that suddendly and without warning all those rounds arrived over the target and detonated in an air burst showering all the exposed infantry from above.. it destroyed entire regiments..

The other little known but key use was in the AAA role against V-1 Rockets.. 90mm batteries on the English coast with prox fuses were deployed along the coast where the rockets had to travel..they were firing in battery.. 4 or more guns at once..and they were often getting the bombs in one volley..

radar-guided triple-A vs. V-1s, 1944:

http://www.youtube.com/watch?v=R-GhqhUKRVU

First time in warfare that robot fought robot.