回来我掌我家的Physics Ph.D. from Yale想想，这简单说，就是交流电，

AC= alternating current, from residential power supply (see details at: http://en.wikipedia.org/wiki/Alternating_current)= "变化的电场产生磁场", while if this material has magnetic property (or even the tool you are using.  I can google more about the mechanism of the tool you used to double check if it's one of the possibilities), then 变化的磁场产生电场...

For some good certified electrician, if you know some of them, they will give you detailed answers too.  I have had a very good house inspector, he understands the tools and the mechanisms beyond the tools both, his report even listed reference articles too. if I have a chance, I will ask him too.