Inside the turbine hall

But in that case, we would have been denied the pleasure of this unrivalled space. It will be open to all: you will be able to walk right through it from west to east, and across it on a higher-level transept from south to north. Once on the riverbank, you will be able to stroll across Sir Norman Foster's surprisingly large new footbridge - another Millennium project - to St. Paul's cathedral. When Bankside was built, London was still low-rise and the power station's tall central chimneystack was seen as a terrible visual intrusion on a skyline dominated by Wren's Cathedral. Today - 40 years after the power station was built - the two buildings carry at least an equal cultural value. Arguably the new Tate carries more clout internationally than St. Paul's.

When the Swiss architects, Jacques Herzog and Pierre de Meuron, won the competition to convert Bankside, there was a fair bit of controversy. Their designs were the ones that appeared to alter the building least. It was only an old power station for God's sake - not even a protected listed building, though a number of conservationists - fogeys and modernists alike - felt that it deserved to be. Scott's other great London power station, Battersea with its four white corner chimneys, lay half-demolished (it still does) and the powers that be did not want another white elephant. Had there not been a recession on, it would have been demolished and replaced by office blocks. Other architects intended to knock it about much more. One contestant even wanted to remove the chimney, on the entirely logical grounds that it was no use and got in the way. But Herzog and de Meuron's subtler approach has paid off. They liked it for what it was - a readymade. Jacques Herzog, clearly a Dadaist at heart, has simply inserted a laughably small, plain fireplace into the chimney on the inside.

The suites of galleries with their rough-sawn oak floors and iron gratings are nice. They're perfectly OK, especially the very tall ones. You can see out of some of them, which is unusual in art galleries. The architects even manage to overcome the modernist tastefulness of it all with some defiantly ugly black staircases and bars. But all that - the contrived art stuff - is a sideshow. Literally so, since it is all on one side of the building. The main event is the immense turbine hall, and this is so mindbendingly good that it makes the Tate's modern collection - rather a thin and patchy one, by international standards - look incidental. Does this matter? Not in the slightest. Art is merely the excuse here. London has a new, world-class, public rendezvous.

Images courtesy of The Tate Gallery