The taxes authorized by this article shall not be imposed unless the board shall have authorized it by ordinance adopted by a vote of two-thirds of all members of the board and the ordinance shall be approved by two-thirds of the votes cast by the qualified electors of the district voting on the proposition at an election called for that purpose. The board, in the ordinance, shall state the nature of the tax to be imposed, shall provide the tax rate or rates or the maximum tax rate or rates, and shall specify the purposes for which the revenue derived from the tax will be used.