When any bonds shall have been issued by a city of the first class for the purpose of constructing or aiding in the construction of a system of waterworks, power plant, sewerage, heating, lighting, or drainage, there shall thereafter be levied annually upon all taxable property of such city a tax not exceeding seven cents on each one hundred dollars for every twenty thousand dollars of bonds so issued, which shall be known as the waterworks tax, power tax, sewerage tax, heat tax, light tax, or drainage tax, as the case may be, and shall be payable only in money. The proceeds of such tax, together with all income received by the city from the payment and collection of water, power, heat, or light, rent, taxes, and rates of assessments, shall first be applied to the payment of the current expenses of waterworks, power plant, heating, or lighting, to improvements, extensions, and additions thereto, and interest on money borrowed and bonds issued for their construction. The surplus, if any, shall be retained for a sinking fund for the payment of such loan or bonds at maturity.