Original Robertson outside frosted antique light bulb with a carbon or drawn tungsten filament, made in England around the turn of the century. This vintage light bulb uses a plaster base cement & has a circular single coil filament. Etched on the glass is ROBERTSON 100-16. It has an early UK standard bayonet cap fitting. Shown glowing at only 50 volts for the photograph. Click on the photo for a bigger image.