So in Cecil’s classic Do “iron fortified” cereals really contain iron filings? column he says “Just don’t try walking through a metal detector afterward,” which I take to be said with the typical Cecil sarcasm, and not as a true statement. But this leads me to ask, just how much iron fortified cereal would one have to consume to cause a metal detector to register you as a potentially dangerous weapon?
Let’s take “Total” as our cereal, and the standard airport metal detector as our standard here. Does anyone have any details on the cereal and the detector that we could use to figure this out?
Well, for starters, you’d need to eat at least enough to exhaust the hydrochloric acid in your stomach, since iron chloride isn’t conductive. Then, we have to assume that the iron powder hadn’t already rusted before you ate it. Then there’s the matter that iron isn’t the best of conductors, as metals go (and therefore, metal detectors are less sensitive to it than to, say, copper or aluminum), and then there’s the fact that you need a circuit for a metal detector to detect, and a powder makes for awfully small circuits. Offhand, I’d say you’d probably need a tightly-packed lump a few centimeters across, in equilibrium with iron-saturated stomach juices. And if you eat anything other than the iron itself (like, say, the cereal that was fortified with it in the first place), that would tend to prevent the iron from being clumped at all. In other words, I don’t think there’s any amount of iron-fortified cereal one could eat to set off a metal detector. You’d have much better luck setting it off with the fillings in your teeth.
In fact powdered iron and laminated cores are used in magnetic circuits such as transformers so that circulating currents in the core are kept as small as possible. This reduces the effect of core loss that is coupled back as a load on the source.