The latter were individually operated in an electrical series "loop" like a string of miniature Christmas lights. A pintype insulator was atop this pole line circuit with a "breaker", "Johnny Ball" or "guy wire" insulator separating each end of the line wire. From there two wires led from each deadend to the pole-mounted series mercury vapor transformer (seen on the pole) that ran each light. There was a provision so that if one lightbulb failed the rest would stay lit. The fixtures shown were quite revolutionary in their day. They were introduced by Westinghouse in 1947 as the first luminaires specifically designed for mercury vapor lighting service. These were known as the Westinghouse OV-20.

The series street lighting arrangement with the poletop insulator and line-break insulator as seen was popular for decades. There still are a few installations like this in service, notably in Los Angeles and San Francisco areas and some other California communities.