The mayor and board of aldermen of any city of the fourth class, upon the assent of the constitutionally required percentage of the voters of the city voting on the question, may, by ordinance, issue bonds of the city in amounts not less than one hundred dollars each, for the purpose of paying any indebtedness of such city, reduced to judgment, which bonds shall run for a period not exceeding twenty years, may carry interest at a rate not exceeding the rate per annum authorized by law, which shall be signed by the mayor, attested by the city clerk, and shall bear the seal of the city.