Movements in the currency market are best described as a random walk: unpredictable, like a drunk staggering home. But whereas most of the time the movements are gentle, swaying like someone who has shared a decent bottle of chianti with a friend, at present they are lurching like a 16-year-old swigging a bottle of tequila. The economies of Europe, Asia and the US would all be at risk from further dollar falls.

The greenback fell close to its all-time low against the euro last Friday, although it has held up against Asian currencies. The level of the dollar affects the sustainability of the world economy's most striking feature: America's current account deficit, now running at about 7 per cent of gross domestic product.