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

I understand they work based on magnetic inductance, but I don’t understand how the detector is able to identify the presence of a metal and its magnetic field?
gpt
In a single coil metal detector, a pulse of current will generate a magnetic field around the coil, which will induce eddy currents in any ferrous objects nearby.  This will cause the ferrous objects to temporarily produce a magnetic field, and the detector quickly switches over to sensing any voltage induced in its coil by *that* magnetic field. 

With multi-coil detectors, any ferrous object in the vicinity of the detector will cause a change in the magnetic coupling between a driven coil and a passive pickup coil.