Each trustee, other than the ex officio members of the board, shall take an oath of office, that, so far as it devolves upon such trustee, the trustee will diligently and honestly administer the affairs of the board, and will not knowingly violate or willingly permit to be violated any law applicable to the retirement system. Such oath shall be subscribed by the trustee taking it, and certified by the officer before whom it is taken, and immediately filed in the office of the secretary of state.