The affairs of the National Trust shall be under the general direction of a board of trustees composed as follows: The Attorney General of the United States; the Secretary of the Interior; and the Director of the National Gallery of Art, ex officio; and not less than six general trustees who shall be citizens of the United States, to be chosen as hereinafter provided. The Attorney General, and the Secretary of the Interior, when it appears desirable in the interest of the conduct of the business of the board and to such extent as they deem it advisable, may, by written notice to the National Trust, designate any officer of their respective departments to act for them in the discharge of their duties as a member of the board of trustees. The number of general trustees shall be fixed by the Board of Trustees of the National Trust and shall be chosen by the members of the National Trust from its members at any regular meeting of said National Trust. The respective terms of office of the general trustees shall be as prescribed by said board of trustees but in no case shall exceed a period of five years from the date of election. A successor to a general trustee shall be chosen in the same manner and shall have a term expiring five years from the date of the expiration of the term for which his predecessor was chosen, except that a successor chosen to fill a vacancy occurring prior to the expiration of such term shall be chosen only for the remainder of that term. The chairman of the board of trustees shall be elected by a majority vote of the members of the board. No compensation shall be paid to the members of the board of trustees for their services as such members, but they shall be reimbursed for travel and actual expenses necessarily incurred by them in attending board meetings and performing other official duties on behalf of the National Trust at the direction of the board.