A city of the first class or city of the second class may enter into a contract or contracts to sell electric, water, or sewer service to persons beyond the corporate limits of such city when, in the judgment of the mayor and city council of such a city not having a board of public works or of its board of public works in such a city having such board, it is beneficial to such city to do so. No such contract shall run for a period in excess of twenty-five years. Such city is hereby authorized and empowered to enter into contracts for the furnishing of electric service to persons, firms, associations, and corporations beyond the corporate limits of such city.