To be effective, an alarm systems can't just ring a bell, although this can sometimes be enough to scare off a prowler. Many people like a silent alarm system. These systems do not ring when someone breaks in, but instead they call the police silently using a connection wired to a central database, and then to the cops. The cops arrive and catch the thieves by surprise, and that way they have no time to get away with your posessions.  

  Recently, I was thinking about it and I decided that I'd just as soon scare the thieves away, get them out of my house as soon as possible. This seemed like a safer move to protect myself and my family, and I realized that if they were running to beat the cops, they wouldn't have time to steal too much anyway. Now, when my alarm system calls the police, it rings also a loud piercing bell, so piercing that it is painful to hear and be in the same room as. This way, if the cops are delayed (and I've heard of cases where it takes the cops a whole hour to get to a house in the middle of a burglary) the thieves are long gone anyway, and the house is peaceful and secure.

Museums have fascinatingly complex alarm system, as does any place that requires high security. I remember once when I was a kid, leaning too close to look at a painting, and setting off an infra-red alarm. Everyone looked at me, and I felt quite embarassed, but in reality, all I was trying to do was to see the painting more closely. Some of these museums (although, fortunately, not the one I was in) even have steel gates designed to stop people from being able to exit once they sneak in and steal something. And in addition to this, they have that good old standby of security: guards patrolling with flashlights and weapons. When you think about it, it is surprising that art works get stolen ever at all