BFO Theory - Basics of BFO operation
Coil Basics (PDF-506K) - This article describes a variety of induction-balanced coil methods.
Coil Parameters - Database of coil parameters
Induction Basics (PDF-156K) - This article describes induction and eddy currents, and their role in metal detection.
Electronic Metal Locators - Don Lancaster's article
How Metal Detectors Work - A white paper written by Mark Rowen and Bill Lahr of White's Electronics
How Metal Detectors Work - by Jeff Tyson, from the "How Stuff Works" web site. Several pages of introductory explanations, including some animated graphics.
Multilayer Air-cored Coils - Equations for calculating the inductance of large-loop coils