These lamps were designed to work on special constant current high volltage AC circuits. They had special transformers with mechanically movable iron cores that responded to the current demands of the entire lamp string. some transformers were designed to operate several strings at once. The lamps were designed such that each unit had a starting resistance that took a part of the current surge when the string was energized. This resistor was cut out as soon as the arc formed. The lamp casing has a special screen part (shown) around the top to keep insects out. only the outdoor street lamps had this screen.[id=298612884] to see a technical explanation of this lamps operation