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A filament is a thin wire made of tungsten that glows brightly when an electric current passes through. Filaments are found in light bulbs. A filament glows because the electrical current raises the temperature of the filament to about 4,000°F (2204°C). The study of lamp filaments can be important in accident reconstruction. In some instances, when a road accident occurs, it is not clear if the lights of a vehicle were on or off. One witness might contradict another one, and this question might be very pertinent to the outcome of the case. The forensic scientist can examine the remnants of the light bulbs and determine if they were on or off by looking at the filament.

Filaments are manufactured by extruding molten tungsten through a die, in a very similar process to the one used to manufacture spaghetti. The filament is then folded in a spiral and takes the general shape of a spring. The filament is supported by small steel arms placed inside the bulb. The filament is also connected at each end to electrical contacts, which will allow the current to pass through. The bulb is usually filled with an inert gas such as argon. If oxygen is present, the filament will burn as soon as it is switched on.

The determination of whether a light was on or off is based on the study of the breakage of the filament. Several situations can occur, each of them leading to a different phenomenon. If the filament was off, it is cold. If a shock is strong enough to break the filament, it will bear a neat break and a clean fracture surface will be present. If the filament was on and broke with the glass of the light bulb still intact, the fracture surface will show evidence of melting and the filament will be completely distorted. The reason is that the shock is fairly violent and when the filament is hot, it is easily deformed due to its spring shape. If the filament was on (hot) and the glass broke, it will burn in the air. Tungsten will react with oxygen to form tungsten oxide. If tungsten oxide is present on the filament or on the glass around the filament, it means that electricity was running through the filament while it was exposed to the air. If the filament was on at the time the glass broke, small particles of glass will be attracted toward the center of the light bulb, due to the slightly negative pressure inside the bulb, and would be deposited on the incandescent filament. At this point, the glass would melt and stick on the filament in the form of spherical glass beads. This could be the only evidence that a filament was on when the glass of the light bulb broke.

Examination of lamp filaments is performed under a microscope, so magnifications of 520 times can be achieved. If a light microscope does not allow the forensic scientist to conduct the examination properly, a scanning electron microscope (SEM) can also be used. The SEM provides the examiner with a much clearer picture of the filament and particularly of the glass beads, if present.

see also Accident reconstruction; Crime scene investigation; Physical evidence.