A city is authorized to enter into contracts for a period not exceeding 40 years for the supply of water, and for a period not exceeding 30 years for the supply of electric power or other public commodity or services. (Code, ss. 704, 3117; 1901, c. 283; 1905, c. 526; Rev., s. 2916; 1907, c. 978; P.L. 1917, c. 223; C. S., s. 2623; Ex. Sess. 1921, c. 58; 1927, c. 14; 1933, c. 69; 1949, c. 938; 1955, c. 77; 1959, c. 391; 1961, c. 308; 1967, c. 100, s. 2; c. 1122, s. 1; 1969, c. 944; 1971, c. 698, s. 1.)