I just checked, and the wording is infuriatingly vague. It is definitely something that is actually referred to as a Faraday Cage, but there is language that implies (to me, at least) that it just uses the same principles, but with mystical grounding structures rather than electrical ones.
You could be right though, so I apologize for my earlier extreme reaction. It was out of line to act like that without checking the book first. Hope I haven't put you off!
...It still does absolutely nothing to prove that "Wizards are electromagnets" fanon theory.
Not a problem. Incidentally, since Harry did use the structure to 'hang' spells off of, one way to interpret it is that the structure was indeed a Faraday cage, and the effect of the spells was to convert any inbound spell energy into EM/electrical energy which a Faraday cage would disperse in a normal fashion. Hence my comment from before that absent more from Jim, it is dependent on how one's own group wishes to handle the issue.
Incidentally, two things. Towards the end of Grave Peril, Harry was being shot at by people using automatic weapons (submachine guns if I can remember) and the they fired with Harry pumping energy into his Shield Charm/Shield spell, the guns themselves began to jam and misfire. This is a clear indication that Wizards mess with more than just things operating on the EM spectrum, which Harry confirms in a conversation with Murph when he states his preference for a revolver instead of a (semi) automatic pistol which he could cause to jam.
Secondly, the hexing rules from the playtest itself also had clear indications that at the upper levels of effect, Industrial Revolution technological items and late pre-Industrial Revolution technological (read: firearms and steam engines) items could be impacted, even unconsciously.
