Quite a lot, not all of it good. Take the Tate's famous Rothko Room; a single, meditative space, hung with a series of Mark Rothko's intense, subtle, richly-coloured abstract canvases. At the old Tate, the Rothko Room was simply a room inside a building. Transferred to Bankside - which is emphatically not just a building - the Room has a different impact. The process of getting to it takes you through this most spectacular of readymades. Consequently your response to the Rothkos is coloured by your knowledge of what lies outside the door of the room. Look at it this way - it is as if both you and Rothko were together in the belly of a whale. In those extreme circumstances, your appreciation of Rothko would be altered, would it not, by the thought of the whale?

The found object: after conversion

The moment you enter this £134m conversion job - preferably strolling down the immense concrete ramp from the wide entrance slot carved out of its western end - you realise that no artist can give you an equivalent experience. The experience is one of scale - of being in a covered space so enormous, you could fly a plane through it. Power stations are often compared to cathedrals, and indeed the original architect of this one, Sir Giles Gilbert Scott, was himself a cathedral architect. He designed both Liverpool's Anglican cathedral, and, by way of contrast, the original red telephone kiosk. But being in Bankside, built in the 1950s, is a qualitatively different experience from being in a cathedral nave.

The floor of the turbine hall - now lowered right down to basement level, to maximise the effect of height - is like an elongated city square. To either side of you, the massive riveted columns of the Lanarkshire Steel Company rise up and up. Overhead, if you're lucky, the electric gantry cranes of Sir William Arrol and Co, Glasgow, will rumble to and fro. Their names - suddenly potent, like Dadaist signatures - are still there to be seen. This great space is literally electric, humming with the noise of the still-active transformer station that occupies the south side of the building. Gigawatts of electricity generated in the Midlands pass through here. It is a readymade sound sculpture: not content with its awesome interior, Tate Modern even has its own special 'found' noise.

This is all very good, but it comes at a price. Anyone who saw the original Cyclopean chunks of generating equipment previously housed in this turbine hall will know that the enormous steel boiler-like sculpture placed here by the veteran artist Louise Bourgeois is a very poor substitute, even if it did arrive on a low-loader lorry just like a real industrial boiler. This does not mean that Bourgeois is a poor artist - only that in this setting, she doesn't stand a chance. The ghosts of the real machines are present. Perhaps their carcasses should have been left, in which case Tate Modern would have turned out a great deal cheaper.