laptop or a cellphone that flips open like a clamshell - Magnetic Reed Switch works

                                                          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 that join together when you push a button and spring apart when you release it. Rocker switches on wall lights push the two contacts together when the switch is in one position and pull them apart when the switch flicks the other way.

Normally open:

In a reed switch, the two contacts (which look like metal reeds) are made from magnetic material and housed inside a thin glass envelope.As you bring a magnet up to a reed switch, it magnetizes the contacts in opposite ways so they attract and spring together and a current flows through them. A reed switch like this is normally open (NO) (normally off), unless a magnet is positioned right next to it, when it switches on:Working of wire contacts when magnet is placed and it is a Normal open connection and Normal close connection is just opposite working of  NO.Take the magnet away and the contacts—made from fairly stiff and springy metal—push apart again and return back to their original positions.

Normally closed:

You can also get reed switches that work the opposite way. The two contacts are normally snapped together. When you bring a magnet up to the switch, the contacts magnetize, repel one another, and split apart, opening the switch and breaking the circuit. Reed switches like this are called normally closed (NC)(normally switched on), and they switch off when you bring a magnet up to them.