What Size Magnet Can ERASE a Credit Card?

Since you edit video and like to see magnetic tracks, you might be interested to know that from the invention of video recording in the mid '50s until the early '70's, 2 inch video recordings were physically cut in order to edit. The location for the cut was found using magnetic developer (fine iron oxide particles suspended in a liquid that would evaporate. The exact location was found with a microscope to see the frame start on the control track. Once cut, it was taped together in a way similar to audio tape. It was hard to do well and most shows tried to avoid the need. The most notable exception was "Laugh In" which used extensive physical cutting along with off-line film editing and film style audio post production to create a unique production style.