Hate to pour water over the idea from an enterprising graduate, Chao Liu, which we report today, for a laptop alarm that screams when disturbed. But I have to say from personal experience that theft alarms can be dangerous.

A few years back I was in New York with an expensive digital camera, and trying out a theft alarm that had been sent in for review. It had two parts: you stuck one on your camera, or whatever you were trying to protect, and kept the other in your pocket.

All was quiet until the two pieces came to be further than a certain distance apart, when a loud alarm sounded. Naturally you had to remember to switch the thing on and off as needed. Naturally I forgot, which may have saved me from injury or worse.

Wandering round the streets of Manhattan I was approached by a street hustler, who gave an entertaining but manifestly dishonest pitch for money. As is the way of these things, it took me a few seconds after he had disappeared to realise that the real purpose of his playacting was to distract my attention. An accomplice had pinched my camera but at least my companion and I had come to no harm.

If the alarm had been primed and sounded there is no knowing how the two thieves would have reacted. In their panic, someone might well have got hurt.