The magnetism on the edge of the MacBook screen, and on the iPad Pro screen, can easily be explained by Apple's long-time use of magnetic switches to turn devices off or put them to sleep when the lid is closed. But as Rita points out, that does not explain the magnetism on the iMac display.
On the theory the magnetism in the iMac is an artifact of the electronics in the display and since the closest thing I have to an iMac is my wife's 27" Apple Thunderbolt Display, I tested it with a paper clip and found magnetic “hot spots” every four or five inches around the entire display frame. Since it would place my life in jeopardy, I elected not to disassemble my wife's display, to absolutely verify my theory. But I am confident that had I disassembled the monitor would have found the magnetic “hot spots” correspond with electromagnetic coils or components of the display. NOTE: I did not find any corresponding magnetic “hot spots” on my more recent LG Ultra Fine or LG
Last edited by joemikeb; 10/03/22 06:30 PM.