Fire Inspectors Test Magnet for smoke & heat

Are you a fire inspector who tests smoke detectors and magnets? Having trouble finding that key spot to set it off? Now you don't need to wonder where it is. Just put this thing up there, and boom, you're done

Here is where you get the magnets:
https://www.harborfreight.com/ceramic-magnet-assortment-16-piece-58066.html