These fixtures were introduced in 1949 by General Electric and operated then-new mercury vapor light bulbs (mostly 400-watt ones). These cutting-edge luminaires and mercury light sources for street and area lighting quickly caught on throughout the United States commencing in the 1950s. Mercury vapor lightbulbs provided twice as much light for the same wattage incandescents and lasted much longer. Many GE Form 109 mercury vapor fixtures were installed throughout Chicago through 1954 (when this fixture design was superceded by a different GE type). Here are two GE 109s interestingly mounted at differing heights looking pretty cool on a downtown pole.