The board of education of Lawrence county may designate one or more school districts within said county (except that no part of the territory embraced within the special school district established under the provisions of the amendment of the Constitution proposed by Act No. 473 of the regular session of the legislature of Alabama of 1949 [Acts 1949, p. 690] shall be included in any such special school district) and may sell the issue bonds in an amount not exceeding one hundred fifty thousand dollars ($150,000) for each such special school district for the construction, improving, adding to, or equipping of a school building, or buildings, within said district. To pay the principal of and interest on said bonds and any redemption premium thereon, Lawrence county may levy and collect an annual tax on all taxable property situated within the special school district with respect to which such bonds may be issued, at a rate not in excess of five mills on each dollar's worth of said property as assessed for state taxation for the preceding tax year; provided, that whenever said tax has produced an amount sufficient to pay the principal of and interest on said bonds the tax shall not be thereafter levied and any surplus remaining therefrom shall be used for general school purposes in said school district. The bonds and tax herein authorized shall be in addition to those heretofore authorized, and no such bonds shall be issued, no such tax shall be levied, until the estimated cost of constructing, improving, adding to, or equipping the school building or buildings to be built in such district, its estimated time of completion, the maximum principal amount of the bonds proposed to be issued, the maximum rate of interest to be paid thereon, and the period over which the bonds to be issued will be retired, shall have been determined and made public by the board of education of said county, and the proposed issuance of bonds and increase in the rate of taxation shall have been authorized by a majority of the qualified electors of such special school district voting upon such proposal at an election to be called by the county governing body for said purpose, said election to be held not less than sixty days after the adoption of this amendment, but at anytime thereafter at the discretion of the said county governing body. The election provided herein shall be called, held, conducted and canvassed, and may be contested, as in the case of three mill school tax elections held pursuant to article 7, chapter 10, Title 52 of the Code of Alabama of 1940. Any bonds issued pursuant to this amendment shall be payable solely out of the proceeds of said tax which may be pledged therefor, but said bonds shall constitute negotiable instruments although payable from a limited source, and said bonds shall be eligible for the investment of trust funds. This amendment shall be self-executing.