In conducting inquiries and investigations authorized under sections 1, 3, and 5 to 7 of this title neither the director nor any member of the United States Bureau of Mines shall have any personal or private interest in any mine or the products of any mine under investigation, or shall accept employment from any private party for services in the examination of any mine or private mineral property, or issue any report as to the valuation or the management of any mine or other private mineral property. Nothing herein shall be construed as preventing the temporary employment by the United States Bureau of Mines, at a compensation not to exceed $10 per day, in a consulting capacity or in the investigation of special subjects, of any engineer or other expert whose principal professional practice is outside of such employment by said bureau.