INTEREST RATE DIFFERENTIALS TOO SMALL, BANKER SAYS Swiss National Bank President Pierre Languetin said a wider interest rate differential between the dollar and stronger currencies was needed to brake the dollar's fall. At a news conference, he said Japan and West Germany could try to stimulate their economies further by expanding money supply, but he added "I'm not so sure it would be desirable if monetary policy became more expansive. "But what would be useful is a greater differential in interest rates," he said.