The round cannister device you see here is the ballast that operated the 400-watt street lamp. It was powered by an automatic photoelectric lighting control (not shown) via adjacent 120V or 240V customer service lines; the ballast's secondary wires led to the mercury vapor lamp inside the fixture.

Sometimes more than one fixture operated from a photoelectric lighting control (often at 3000 watts capacity). Some utilities used a blue, yellow or green porcelain insulator to identify such dedicated control wires.