I've noted that there has been some discussions on going relative to the subject matter. I recalled a link that I had in my archives for sometime and wanted to share it with you all. For those that has been around awhile you have probably already known most of this though I am sure some of our newer members and especially those new to the hobby could probably benefit by it. In any case it is a good read.
http://home.howstuffworks.com/metal-detector2.htm
http://home.howstuffworks.com/metal-detector2.htm