In short, they generate a magnetic field with a coil which then passes over the metals. These generate magnetic fields of their own which the detector, well, detects.
The long answer involves utilizing oscillator fields and eddy currents as illustrated by great Scottish physicist James Clerk Maxwell’s equation of electromagnetism and… never mind, we’ll stick to the short answer.
Seriously though, to give some credit where it’s due, Maxwell was a genius. Even if the science is over our heads, we should all be thankful for the man. Einstein said there is only one person more influential in science and modern life than Maxwell. That person is Sir Isaac Newton, the guy who figured out how gravity works. Plus, Maxwell had a rockin’ beard.
James Clerk Maxwell (1831 – 1879)
Back to the aluminum can though. A lot of folks mistakenly believe that such electrical currents only work with ferrous metals like iron and Steel. However, this isn’t actually the case! Technically all metals are magnetic. Many are only slightly magnetic or are magnetic at really high or low temperatures. This means that these metals react to the electrical and magnetic fields generated by metal detectors. As such, yes, McConaughey actually would have been able to detect a beer can with his metal detector! Probably the only believable or realistic part of that whole movie.
Have a great summer, have fun, and if you make any of these things, please let us know on Facebook or Twitter!