The Iron Bridge at Coalbrookdale, opened in 1779, had proved that cast iron could span a river. Its 30-metre arch was small but transformative - the first major bridge in the world made of cast iron, an icon of the early industrial revolution. The Wearmouth crossing took the idea and doubled it. Rowland Burdon, Member of Parliament for County Durham, sponsored and patented the design; the architect Thomas Wilson directed construction. The bridge spanned 240 feet - over twice as long as Coalbrookdale - and weighed only three-quarters as much. At the time of its opening it matched the collapsed Trezzo Bridge in Italy for span, but unlike Trezzo it stayed up. Nikolaus Pevsner later called it 'a triumph of the new metallurgy and engineering ingenuity, of superb elegance'.

Thomas Paine's involvement is the kind of historical thread that sounds invented but isn't. After his American adventures, Paine spent the late 1780s on cast-iron bridge schemes, partly out of engineering interest and partly because he believed iron bridges would change the world. The Paddington demonstration span - 110 feet long, exhibited from 1789 - was a proof of concept that didn't find a buyer. When Paine fled to France in 1792 amid government suspicion of his radical writings, the prototype was returned to Walker's foundry at Rotherham. The Wearmouth bridge was cast at the same foundry. The chronology and the geography fit. The conservative parliamentarian Burdon and the revolutionary Paine never met over the project; their iron met instead, melted down and re-cast into the ribs that carried Sunderland's traffic for 130 years.

The foundation stone was laid in September 1793. Construction took three years. The bridge opened to traffic on 9 August 1796 at a total cost of about £28,000. According to the plaque on its successor, it 'proved to be a catalyst for the growth of Sunderland' - and the claim is justified. Before the bridge, Monkwearmouth on the north bank and Bishopwearmouth on the south were connected only by ferry, with the nearest road crossing 12 miles upstream at Chester-le-Street. A bridge in the middle of the town transformed everything. Tolls were charged at first - the standard model for funding privately patented infrastructure - and pedestrian tolls were abolished only in 1846. The bridge worked, beautifully, except for one problem nobody had anticipated.

In 1805, the sun caused a problem. Heat expanded the cast-iron components unevenly, and some of the cross tubes fell out of the structure. Repairs followed. The bridge stayed in service, but its long-term durability was a concern. From 1857 to 1859 Robert Stephenson - son of George, builder of railways - stripped the bridge back to its six original iron ribs and raised the abutments to level the roadway. This second-generation Wearmouth Bridge reopened in March 1859 and remained in service until 1927, when increasing traffic forced its replacement. The current bridge, built 1927-29 around the old one to keep the road open, stands on the same site today. Two of the ribs Paine may have cast, Burdon paid for, Stephenson stripped down and Sunderland crossed for over a century are gone now. But the elegance Pevsner praised had a long innings.