To pay for construction, operation and maintenance of said works and expenses preliminary and incidental thereto, the board is hereby authorized to enter into contract with the United States or any agency thereof, providing for payment in installments or to issue negotiable bonds of the district. If bonds are authorized, the same shall bear interest payable semiannually, and shall be due and payable not less than ten nor more than fifty years from their dates. The form, terms and provisions of said bonds, provisions for their payment and conditions for their retirement and calling, not inconsistent with law, shall be determined by the board and they shall be issued as hereinafter provided in payment of the works, equipment, expenses and interest during the period of construction. Said bonds shall be executed in the name of and on behalf of the district and signed by the president of the board with the seal of the district affixed thereto and attested by the secretary of the board. They shall be in such denominations as the board shall determine and be payable to bearer and may be registered in the office of the county treasurer of the county wherein the organization of the district has been effected, with the interest coupons payable to bearer, which shall bear the facsimile signature of the president of the board. Such bonds shall be exempt from all state, county, municipal, school and other taxes imposed by any taxing authority of the State of Nebraska and shall not be sold at less than par and accrued interest.