New Federal Reserve chair Kevin Warsh promised to preserve the Fed 's independence as he was sworn in at the White House on Friday. It was an unusual step, since Fed chairs are not normally sworn in at the White House, and presidents usually don't attend, given that the Fed is meant to be politically independent. At Warsh's confirmation hearing, US President Donald Trump insisted the incoming central bank chief would be "totally independent." But some, particularly Democrats, have questioned how independent Warsh will be as head of the world's most powerful central bank. Among them is Democratic Senator Elizabeth Warren, who described Warsh as Trump's "sock puppet” during his Senate confirmation hearing in April. Stressing that independence requires courage, Warren asked Warsh: "Let's check out your independence and your courage. We'll start easy. Mr.…