 The advantage of the coiled coil is that evaporation of the tungsten filament is at the rate of a tungsten cylinder having a diameter equal to that of the coiled coil.The coiled-coil filament evaporates more slowly than a straight filament of the same surface area and light-emitting power.As a result, the filament can then run hotter, which results in a more efficient light source while lasting longer than a straight filament at the same temperature.