How Automobile Light Bulbs Work
While light bulbs might seem trivial, simple things to talk about, relatively few people really know the complex physics process that takes place when a lamp turns on. These commonplace items haven't significantly changed since the days of Thomas Edison, but they are nonetheless marvels of science. We thought it appropriate to explain this process in a little detail.
Mechanically, light bulbs consist of a metal base, which itself consists of a screw thread contact (attached electrically to one side of the filament), insulating material and an electrical "foot" contact (the little brass bulge on the bottom which is electrically connected to the other end of the filament.)
The metal contacts at the base of the bulb are connected to two stiff wires that go to the center of the bulb and, in turn, hold the filament. The bulb itself is the glass housing that not only shields the filament from oxygen in the atmosphere but also holds in an inert gas, usually argon. More about that later.
The filament is the part of the bulb that does the work to create light. It is made up of a long, extremely thin (about .01 inch) length of tungsten, a very versatile metal. The typical filament in a household bulb is over six feet long and is tightly wound to form a double-coil.
How It Works
When electricity is passed through the bulb the electrons (current) vibrate through the filament, creating very high temperatures — upwards of 4,000 degrees F. Why so high a temperature? Well, this temperature is needed to cause the atoms in the filament to gain energy and then to emit photons in sufficient quantities to bathe the area in a useful amount of visible light. Most of the photons emitted by a bulb are infrared, by the way, which is invisible to humans. Infrared light is pure heat and that's why bulbs get so hot.
Anyway, when you heat metal to such high temperatures it wants to melt! Tungsten is one of the only widely available metals that can withstand such temperatures, but in the presence of oxygen it will catch fire and burn itself up. This is why the bulb is evacuated and filled with an inert gas.
Why not save money and just leave a vacuum in the bulb, you ask? Well, if you do that (and early bulb manufacturers did so) the tungsten atoms vibrating at such extreme temperatures will detach and fly around the inside of the bulb. Eventually, these vaporized atoms deposit themselves on the inside of the glass and the filament disintegrates, causing failure of the bulb. The inert gas lengthens the life of the bulb by acting as a "reflector" of tungsten atoms. The metal's atoms bounce off the argon atoms and migrate back to the filament. Ain't physics great?
How Bulbs Die
We all know that bulbs don't last terribly long, in spite of their relative reliability and cheap cost. They eventually fail due to one of four modes:
Breakage of the glass bulb accounts for a small portion of failures, especially in motor vehicles. It's just one of those things that happen every once in a while.
Sealing Failure occurs when the bulb's atmospheric seal is broken and oxygen enters the bulb. The filament burns up instantly. Such failures occur when bulbs are screwed into sockets too tightly.
Long Term Failure occurs when the filament eventually becomes so fatigued that its electrical resistance increases to the point that current won't flow. The inside of the bulb gets very dark and the electrical contact on the base starts to burn.
This is the most common failure mode of bulbs and the one we're all familiar with. As soon as the switch is turned on the bulb flashes brightly and then fails. The reason most bulbs fail in this manner is thermal shock. When the switch is turned on, full current suddenly hits the filament at the speed of light. This sudden, massive vibration causes the filament to wildly bounce (if you saw it in super-slow motion photography you would see the filament jump up and down in a sine-wave, slowly decreasing in size). This mechanical movement causes metal fatigue (just like bending a paper clip until it breaks) that results in breakage of the filament. That's why lights left on always last a long time.
Bulbs in automobiles have to suffer extremes of temperature, vibration and corrosion of electrical contacts from weather. It's a tribute to good engineering that they last as long as they do and cost so little. Now maybe you understand them a little better.