They say Michelangelo didn’t need one - the great artist could draw a perfect circle freehand. But for the rest of us, the compass has always been an indispensable item in our pencil case.

Other items may change with the years - witness the evolution of the fountain pen, which led to the rollerball pen and the trendy gel pen - but the humble compass has stayed much the same. Some of us even have the scars to prove it. Although the introduction of plastic parts and a broadened cross-section means it can’t penetrate the flesh too deeply, the stabbing tip still remains. The screw at the end, which should also keep the whole thing tight, inevitably works loose, making a perfectly drawn circle as hard to pin down as the compass used to create it.

Bill Harper, who worked as a maths teacher for two years in Liverpool comprehensives, saw children struggling with the traditional school compass - creating wobbly and inaccurate circles - and thought about alternatives. “The idea for a new design - a rotating template with holes in it - came to me while I was on teaching practice,” he says.

Over the years he thought about replacing the hinged compass with a plastic template design. But it was only when he changed careers that he could take the project further. Ironically, as a station officer in the fire brigade, he had more time to experiment with his idea than he had as a teacher. He built a prototype and decided to manufacture his invention.

Using the radar graphing facility on his Excel spreadsheet, Bill Harper created a precise spiral, which he printed on acetate. By cutting out the spiral and adding holes, he created his first disk compass. When he approached various businesses, including Helix - the brand leader in compass manufacturing - he discovered the plastic moulds were expensive to mass-produce - he was quoted pound;15,000 to create the basic tooling.

Undeterred, he tried other manufacturers, eventually linking up with a Blackburn company, Mamp;G plastics. With the first classroom-ready models available, Mr Harper set off to change the face of compass use in British schools, having drastically reshaped a design that has been with us since the Greek philos-opher Thales invented it around 2,500 years ago.

Mr Harper took his first batch to regional mathematcal conferences and festivals. At one such event in Birmingham, Gill Morgan from Huyton-with-Roby CE primary school in Huyton, Knowsley, saw the compass in action. After hearing the disk had also been recommended by her numeracy adviser, she persuaded the school to buy a set to use as common resource and safe maths toy for Years 4 to 6.

On a visit to the Merseyside school, Bill Harper gives a Year 6 class a demonstration in the use of his disk. Using an overhead projector, he shows pupils how to draw circles, then they have fun trying it themselves.

The speed advantage comes from no longer having to remove the compass from the paper. Gone is the tricky step of setting a new radius against a ruler. Once the students have rattled off a series of tidy circles, a few more difficult constructions follow, including drawing a face. The class happily add arcs and extra circles to create the features. “It looks a bit like a clown,” I say to a pair of Year 6 girls. “Oh, it’s meant to be you,” they reply.

Although the compass is large - 20cm in diameter - precise circles can be drawn with any chosen radii - from 1mm to 100mm, in 1mm steps. Children no longer have to remove the compass from the paper or set the radius against a ruler. And, claims the inventor, the disk is accurate to 0.5mm. It also works as a 360-degree protractor, with well-spaced graduations on the circumference.

“One advantage is that you can’t lose any of the bits,” says supply teacher Elizabeth Whipp. The children are also enthusiastic about the compass. “You have to hold the old one still - it always wobbles for me,” says a newly confident pupil. Parents seem equally impressed, with one dad pointing out that compass conflict will be a thing of the past.

The disk compass has the approval of examination boards, and recently won a sponsored award from the Merseyside Innovation Centre. You can only buy the compass via mail order, but Bill Harper hopes high street retailers may soon stock his product. “Children, teachers and advisers like it, and they think it will replace the traditional geometry set,” he says.

Circle Scribe Disk Compass is suitable for children in Year 3 and upwards. It costs pound;3.51 (plus pamp;p) or pound;79.31 (plus pamp;p) for a pack of 30. Also available is the booklet, Fun, Art and Geometry, which costs pound;5.50. These are available from Circle Scribe, PO Box 27, Huyton L36 4RS. Tel: 0151 289 5681. Website: www.circlescribe.com