Re: how metal detector detects explosives,etc.?

Thank you Amar for you question. I'll answer part of the question with a 
couple of links to just how metal detectors work. http://www.howstuffworks.com/metal-detector.htm http://en.wikipedia.org/wiki/Metal_detector

Essentially, they rely on electromagnetic induction. You have a coil 
which generates an electromagnetic field. If a conductive, or partially 
conductive, material is close enough to that coil the field induces eddy 
currents (electrical) in that material which in turn generates its own 
magnetic field. So on your metal detector you also need another coil (a 
magnetometer) to detect the induced field. The key is having the 
sensitivity you need but also rejecting a lot of clutter. The links above 
can provide much more detail.

But you're question is a bit deeper - how do you detect explosives. Dogs 
are very good, able to detect the scent of explosive material. There is a 
mechanical analog of this as well (known as an ion trap, among other 
similar technologies). 

So how do metal detector detects explosives? From what I can tell, they 
would only detect metal components - something perhaps contained in the 
encasement or triggering of the device. It will also tend to pick up 
other metal debris, so likely would need to be combined with other 
technologies to reduce false alarms. But the process would still be to 
induce eddy current in conductive materials linked to the explosive 
device, and then detect the induced magnetic field.
 http://en.wikipedia.org/wiki/Explosive_detection http://maic.jmu.edu/research/primer/landmines101/05rnd.htm