A.C. magnetic field (ACMF) sources and their EM interaction mechanisms are discussed in this installment, which continues our July/August series on preventing and resolving site magnetic field problems. This article carries forward the theme of a primer on the basic physical causes and effects of A.C. magnetic fields.with special emphasis on the manner in which such fields may affect site EM instrumentation. Experience has demonstrated that a basic understanding of the various aspects of ACMF sources will often prove helpful in identifying the sources which actually pose a threat to EM site operations.