In the aftermath of the Philadelphia Police Department's latest scandal — the arrest of a 26-year veteran on charges of stealing cash from a bar — few would argue with Police Commissioner Charles H. Ramsey's announcement Friday that the department needs to find a way to weed out corrupt officers. Jerry Ratcliffe, a criminal justice professor at Temple, said that in addition to looking at ways to prevent corruption, the department should evaluate how it investigates corruption. "The difficulty for any police department is that you can't actively discipline someone until they've been found to do something wrong," he said. "So it's hard to be proactive."