Making the coil
1 Since a thin, straight wire has poor emission characteristics and it is difficult to fit into the lamp bulb, the wire is wound into the shape of a coil using automated machines that resemble high speed bobbins. To make a round-core filament, each tum is spirally laid adjacent to the next one on a cylindrical rod. A rectangular rod is used for a flat-core filament. For a double filament, the wire is first wound into a very fine primary coil, and this is then wound once more around a second, thicker core. A large amount of wire can thus fit into a very small space.