Special Report on the Motor Show (10): Keeping track of the thieves: A new device will alert the police when a vehicle is stolen, reports Tony Bosworth

IF you're a professional car thief or one of those young men who has a penchant for attempting handbrake turns in someone else's vehicle, beware. That car you've got your eye on could soon be fitted with a device that will have a police car on your tail inside 30 minutes.

A security system to be made available in Britain in about two months has proved so successful in fighting car theft in America that car owners fitting it get a 35 per cent discount on their insurance premiums.

The device - called Lojack in the US, but which may have a different name over here - comprises a transmitter that can be fitted in any of 40 locations inside the car. If the vehicle is moved and the device is not disarmed, it emits a constant signal that can be picked up by a police receiver. The car can then be tracked and retrieved even if its ignition is switched off.

Lojack has already been endorsed by the Association of Chief Police Officers, and police forces around the country have been assessing its benefits.

The device will cost about pounds 300 to fit into a car. The head of the Met's Stolen Vehicles Squad, Detective Superintendent Robert Melrose, says: 'I have some doubts about your average car owner in Camden wanting to spend that sort of money.

'However, it will enable us to cut down on the professional car theft business because the signal can be picked up even when a vehicle is crated and ready to be shipped out of the country.'

From the thief's point of view there's very little point in stealing a car fitted with Lojack because even if the police do not have a car receiving the Lojack signal in the area when the car is stolen, it could take hours for the thieves to find the transmitter within the car, during which time it will continue to operate, giving police time to get a fix on its location.

For those who don't want to spend pounds 300 on Lojack, Philips is launching a new range of alarms and immobilisers, which it claims will stop false alarms.

The problem with most alarm systems is that they bathe the interior of the car with ultrasonic waves that trigger the alarm when broken, such as when a a window is smashed.

Unfortunately, the waves can also be disturbed by air forcing its way in through the car's air vents when heavy traffic passes, or if the temperature inside the car alters.

The Philips Tri-Sonics system is said to be more competent and senses not only air movement, but also the speed and mass of movement. It also adjusts to changes in temperature. The four alarms in the range, costing from pounds 70 to pounds 250, also feature engine immobilisers and a radio key that is aimed at beating 'grabbers'.

A grabber is a device about the size of a small paperback that copies infra red key codes. When you blip the infra red key at your car to switch the alarm off, the thief, parked nearby, will use the grabber to copy your code. He then follows you home and when your car is unattended he uses the grabber again to open the doors and deactivate the alarm.

The Tri-Sonics system beats the grabber by constantly changing the code so that although the thief may record the code you used the first time, that code will not necessarily be the same by the time you park it again.