The Magnetic Latch

I reached the bottom landing where the concrete floor met the heavy metal exit door. Near the base of the frame, a round electromagnetic lock held the door firmly flush against the jamb. A tiny red diode glowed inside its brushed aluminum collar. Holding my miniature phone in my palm, I moved closer, feeling the faint, steady warmth radiating from the energized coil. When I held the flat plastic edge of the phone against the metal cylinder, the subtle magnetic field tugged gently at its corner with a muffled, quiet click. It holds the whole massive doorway shut using nothing more than silent power.