Any incorporated city or town in this state may, by a majority of the voters of such city or town, voting at an election to be held for that purpose, be allowed to become indebted in a larger amount than that specified in Section 26, for the purpose of purchasing or constructing public utilities, or for repairing the same, to be owned exclusively by such city or town, or for the purpose of constructing, reconstructing, improving or repairing streets or bridges. Provided, that any such city or town incurring any such indebtedness requiring the assent of the voters as aforesaid, shall have the power to provide for, and, before or at the time of incurring such indebtedness, shall provide for the collection of an annual tax in addition to the other taxes provided for by this Constitution, sufficient to pay the interest on such indebtedness as it falls due, and also to constitute a sinking fund for the payment of the principal thereof within twenty-five years from the time of contracting the same.