No Bible was used when Fdr took the oath of office.in 1933

Since there is supposed to be a separation of Church and State, I don't understand why a Christian Bible is used in the oath of office, I think a president should take an oath on the Constitution of the U.S. When I was called for jury duty years ago, I told them that taking an oath on a Bible had no meaning for me, it would be better if they used a copy of Harry Potter.