Quad-Loop Large Exposition Filament Bulb Featuring a clear finish construction with small glass tip on top of the bulb, this 1900's replica is ideal for exposed socket lamps. The quad-loop tungsten filament originally used by Thomas Edison enhances the nostalgic ambiance of this bulb. This A21-shaped Victorian loop bulb is the middle size between the A19 which is smaller and the A23 which is larger. All three sizes fit the standard medium base (E26) socket.