These fixtures were pretty common in their day. Typically they operated in electrical series, like a string of Christmas tree lights. A provision was made within the bulb's socket for circuit continuity when a lamp was broken or burned out. By 1920 most incandescent street light fixtures for lower level illumination (such as along residential streets) were designed with a porcelain fixture head. These were much more durable and provided superior insulation. Series street lighting circuits were still widespread in the United States through the 1960s.