In case any such bonds shall be issued for the purpose of paying off any such judgments or decrees already rendered, or which may hereafter be rendered, on account of indebtedness arising from the purchase or acquisition by such city or town of any waterworks, gas works, electric light works or property from the ownership whereof by the city or town an income or revenue may be derived, all such income or revenue from such waterworks, gas works, electric light works, or property, after paying the running expenses, the necessary cost of repairs or reasonable enlargements or extension, shall be set aside and used, first, for the purpose of paying interest matured or maturing upon such bonds during the fiscal year for which such income or revenue is received, and the residue of such income or revenue shall constitute a sinking fund to meet the principal of such bonds when the same shall become due, and shall be kept and invested and managed as sinking funds of such city or town are required to be kept and invested and managed by the laws of this state and the charter of such city or town.