Any municipality which acquires, constructs or extends any public utility and desires to raise money for such purposes may issue mortgage bonds therefore beyond the general limit of bonded indebtedness prescribed by law; provided that such mortgage bonds issued beyond the general limit of bonded indebtedness prescribed by law shall not impose any liability upon such municipality but shall be secured only upon the property and revenues of such public utility, including a franchise stating the terms upon which, in case of foreclosure, the purchaser may operate the same, which franchise shall in no case extend for a longer period than twenty years from the date of the sale of such utility and franchise on foreclosure.