An Electrical magnetic door switch is like a drawbridge in an electric circuit. When the switch is closed, the "bridge" is down and electric current can flow around the circuit; when the switch opens, the "bridge" is up, and no current flows. So, the purpose of a switch is to activate or deactivate a circuit at a time of their choosing.
Most of the electrical switches we encounter are ones we control ourselves. If you want light in a room, you flick a switch on the wall. Want to watch TV? Turn on the switch. Want to listen to your iPod? Push the wheel at the front and that activates a switch that turns on the power. But sometimes they want electrical and electronic circuits to be activated in other ways.
Suppose you want to wire up a bank safe, so it triggers an alarm whenever the door opens. How would that work in practice? You would need electrical contacts on both parts of the door frame so that when the door opened the circuit would be broken, triggering the alarm. But think how tricky it would be to make a reliable electrical connection on a door frame. What if you painted over it? What if it got dirty? And wouldn't it be so obvious to a thief that they would be able to disable it quite easily? There are lots of ways in which electrical contact could be rendered inactive and useless. This is where a specialty switch can help.
In a typical specialty switch, the two contacts (which look like metal reeds) are made from a ferromagnetic material (that means something as easy to magnetize as iron), coated with a hardwearing metal such as rhodium or ruthenium (to give them long life as they switch on and off), and sealed inside a thin glass envelope filled with an unreactive gas (typically nitrogen) to keep them free of dust and dirt. Sometimes the glass has an outer casing of plastic for even greater protection. Typically, the contacts are made from a nickel-iron alloy that is easy to magnetize (technically, we say it has a high magnetic permeability) but do not stay that way for long (we say it has a low magnetic retentivity). They take some time to respond to changes in the magnetic field (we say they have quite a bit of hysteresis)—in other words, they move quite slowly and smoothly. Generally, both contacts move (not just one), and they make a flat, parallel area of contact with one another (rather than simply touching at a point) because that helps to extend the life and reliability of the switch.
Although most specialty switches have two ferromagnetic contacts, some have one ferromagnetic contact and one that is non-magnetic, while some (like the original Elwood specialty switch) have three.