The Newgate Street Clock will be unveiled by the Lord Mayor of London on May 31 at 11.15 and it is thought to be the only public clock in the world with “a wandering hour” dial.

Joanna Migdal, the clock’s designer, explained how the ingenious mechanism, produced by Smith of Derby, works. The hour appears to rise in the upper aperture of a fixed steel dial and travels slowly round in a clockwise direction passing the sectors indicating the minutes past each hour as it goes. Before it sets, the next hour disc, until then out of sight, rotates into view to indicate the next correct hour. As each hour sets, the next rises.

The “wandering hour” was, in its day, a rival to the traditional concentric-hands clock but is technically very complicated. For a while there was furious competition between them, rather like the struggle for supremacy in the video recorder market. The wandering hour lost, but one of the reasons for choosing the system for the Newgate clock is that one of London’s most celebrated watchmakers, Joseph Windmills (Master of the Worshipful Company of Clockmakers in 1702), had his workshop in near-by St Martin-le-Grand.

Migdal’s clock was built by Smith of Derby, one of the very few English manufacturers able to handle the ingenious modern mechanism which includes LEDs and satellite correction.

After studying at the Ealing College of Art, she obtained a seven-year apprenticeship at the Hindhead studio of the sculptors Edwin Russell and Lorne McKean. It was, she says, “a life-changing experience. Edwin was so thorough, so focused. He insisted that you should think of nothing but your work. His influence and his words stay with me.”

While working as a journeyman she stumbled upon what has become her life’s obsession: designing and constructing sundials and clocks.

It is a precise art, which involves sculpture, metalwork, engineering, lettering and mathematics. She has now spent more than 25 years designing, engraving and making sundials for clients all over the world. Last year she designed the sundial for Tavistock Square to commemorate the bus bombing there in July 2005. The dial is positioned so that at the exact moment of the explosion on the double-decker bus the sun begins to cast a shadow across the dial. Migdal remembers that “it was an extremely emotional moment, watching the dial darken with relatives of the victims and survivors, exactly a year to the second after the explosion”.

The Newgate Street Clock, a gift of the clockmakers of the City to celebrate the 375th anniversary of their worshipful company, has already started “ticking”. It has a two-metre stain-less-steel dial, and although the inspiration for its mechanism comes from the start of the 18th century and Joseph Windmill’s workshop, any extra wandering in its accuracy will be corrected by satellite radio signals.