Magnetic Reed Switch Interesting aspect: If you've got a laptop or a cellphone that flips open like a clamshell, you've probably noticed that it senses when you open and close it and switches on or off accordingly . But how does it know? Some kind of switch wired to the hinge so it can detect the opening and closing movement? If that's what you think, you're at least half right! Think about it more carefully and you'll see a standard switch would be quite tricky to wire up in that way—and probably quite unreliable too: all that opening and closing would quickly wear it out. So, instead, many laptops and phones use an inexpensive and very reliable device called a reed switch that turns on or off when a magnet is nearby. Intruder alarms and model railroads often use them too. Let's take a closer look at how they work! What is a reed switch? An ordinary switch has two electrical contacts in it t