§ 9. Oath of office; eligibility of commissioners and officers. Each commissioner and each person appointed to office by the chairman shall, before entering upon the duties of his office, take and subscribe the constitutional oath of office. The oaths of office other than those of a commissioner shall be filed in the office of the commission. No person shall be eligible for appointment or shall hold the office of commissioner or be appointed to, or hold, any office or position under the commission, who holds any official relation to any person or corporation subject to the supervision of the commission, or who owns stocks or bonds of any such corporation.