In case of the death, resignation, sickness, absence or inability of the head of any of the executive departments of cities of the second class other than the department of city controller, the mayor or the head of the affected department shall designate in writing, filed with the city clerk and with the city controller, a deputy to act for such department head, and to exercise the powers and discharge all duties of such department head during said absence or disability or until a new director is appointed, except the powers of appointment and dismissal of employes. The mayor or the head of the affected department may revoke any such designation at any time without cause by a revocation in writing, filed with the city clerk and the city controller. Such deputies shall serve without additional compensation and may be required, by ordinance, to give bond in such amount as council may fix.