The decision has been Mr Bernanke's major test to date

Here is the text of the announcement from the Fed.

The Federal Open Market Committee decided today to lower its target for the federal funds rate 50 basis points to 4.75%.

Economic growth was moderate during the first half of the year, but the tightening of credit conditions has the potential to intensify the housing correction and to restrain economic growth more generally.

Today's action is intended to help forestall some of the adverse effects on the broader economy that might otherwise arise from the disruptions in financial markets and to promote moderate growth over time.

Readings on core inflation have improved modestly this year. However, the Committee judges that some inflation risks remain, and it will continue to monitor inflation developments carefully.

Developments in financial markets since the Committee's last regular meeting have increased the uncertainty surrounding the economic outlook.

The Committee will continue to assess the effects of these and other developments on economic prospects and will act as needed to foster price stability and sustainable economic growth.