J K ROWLING would be proud. Microsoft has completed trials of a Harry Potter-style tracking device that can monitor the movements of family members through their mobile phones.

The software giant’s Whereabouts Clock features a screen that shows the approximate location of an individual - such as “school”, “work” or “gym”.

Microsoft has just finished field trials of the system, which was installed in the homes of several families living near the company's research laboratories in Cambridge. It is now looking at how to market the product.

Richard Harper, who heads Microsoft’s socio-digital systems research group, said: “We want it to give enough information to be comforting without giving so much that people feel they are being watched.”

The system is reminiscent of a device in Rowling’s novels. The parents of Ron Weasley, Harry’s friend, own a magic clock with a hand representing each family member. The hands point to locations on a dial including “home”, “work” and “mortal peril”.

The Microsoft device works by identifying which cell of a mobile phone network a family member is in at any given time. When users set up the Whereabouts Clock they have to give each cell a name, such as “work” or “school”, by downloading software onto their phone. The device recognises their location each time they enter that cell.

Users who wish to conceal their location can switch off the application. If they want to provide more precise information, they can text a message that will appear on the clock face.

The system is being seen as Microsoft’s riposte to Google Latitude, under which users have their exact location pinpointed on Google Maps.

The Latitude system, which also works by tracking mobile phones, has a number of privacy controls.

Janice Day, 46, a teacher in the Cambridge area, tested the Microsoft clock for a month with her family.

She said she had changed her attitude to being under surveillance: “I didn’t like initially everyone knowing where I was - and I thought, Janice, you hypocrite. But by the end I was actually quite happy about it.”

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