The most basic device that any security burglar alarm system has is the magnetic contact. If you are to be purchasing one of these systems or maybe you already have one and don't know how to start installing it, you will notice that the system will have a few of these relatively small devices.
These devices are responsible for activating the alarm if a window or a door in you home is opened. There are two ways that a magnetic contact may function. They can either be activated when the two parts of the it make contact or when contact between them is interrupted. When contacts activate the alarm when they are split apart it is called a closed system and when they trigger the alarm when they get close together it is called an open circuit. Both of these two types of technologies basically do the same thing. The only difference is in how these devices are to be installed. However, most people will probably find that close circuit devices are easier to install.
Your security burglar alarm's magnetic contacts should be placed on areas that are more vulnerable to burglars. You need to examine you home before you go out to buy an alarm kit in order to make sure that you choose one that is appropriate for your home. There are many things that can really go wrong when you buy one of these system and one of them is not having enough contacts to secure all the important entrances in your home. What good would an alarm system would be if the front entrance is monitored while the house's back door is not?
As you can see magnetic contacts are the bread and butter of your home security alarm and are the simplest and most effective device of all the different sensors that may be included in a home burglar alarm.
More info at: Security Burglar Alarm System

