system
Given the following post, respond as if you're commenting on Reddit:
human
How do metal detectors work?

To clarify, why does not all metal set off a detector? Such as zippers on jeans, or metal buttons? Do these detectors not detect certain kinds of metals, or do they just need at least a certain amount?
gpt
A metal detector makes a varying magnetic field in an area using an electromagnet. Conductors such as metal will temporarily block magnetic fields from entering them due to the skin effect. The driver used to power the electromagnet will be affected due to the presence of large amounts of metal, since it has less space to fill with magnetic field. The power being supplied to the electromagnet will be noticable different, but the amount of difference depends on the conductivity and volume of the material, as well as how close it is to the electromagnet. The distance issue is why an airport security officer will sometimes use a hand wand, since it can be placed much closer to the body.

Conductivity varies a lot between metals. The skin depth is proportional to the square root of conductivity. Stainless steel is 45 times less conductive than copper, so the thickness of stainless steel that is "invisible" to a metal detector is 6.7x more. These thicknesses will still be incredibly thin, at 1MHz the skin depth of stainless steel is 0.5mm.