Long Version: The missing term is called the Sharma transduction tensor, also known as the thaumic flow transduction potential: the amount of nearby matter and energy which is enchanted and/or arranged in a mystically significant way. Flowing electrical lines will contribute to this factor. So will static or charmed magic rings, static or charmed staffs, electromagnetic waves, large electrically neutral masses and, in the smallest quantities, all other mass-energy in the universe out to an infinite range, give or take the inverse square law.
Short Version: A Wizard Did It.
I should have known the Terrator didn't mean us any harm when the Sword of Omens didn't obey me! And anyway, it was just plain stupid to assume it might be bad just what the fuck am I talking about?!