The authority to make or conduct any examination, investigation, or hearing, including the authority to administer oaths and to subpena witnesses, and to take their testimony may be delegated by the commissioner to any deputy, investigator, or auditor appointed by him for that purpose. The appointment shall be made by an instrument in writing, signed by the commissioner. Upon any examination, investigation, or hearing the instrument shall be produced by the deputy, investigator, or auditor at any time upon demand.