They warn and inform road users

While headlights are arguably the most important lights on a vehicle, the taillights, brake lights and indicators also have vital functions. They make sure other road users can see us in poor light and also inform them of our intentions.

While headlights are arguably the most important lights on a vehicle, the taillights, brake lights and indicators also have vital functions. They make sure other road users can see us in poor light and also inform them of our intentions.

These are much more basic, low-wattage lights, using simple bulbs with either one or two tungsten filaments in a clear glass envelope, filled with an inert gas to exclude oxygen.

As we saw last week, the tungsten wire forming the filament will slowly vaporise, as will be evident from a gradual blackening of the glass envelope. Due to this, and a build-up of road grime on the lens surface, the lights will gradually become dimmer. Eventually the filament will either break or the light will get so dim as to be useless.

To forestall this, we should keep an eye on these lights for the early signs of failure. The problems to which they are prone are easy to identify and rectify. If a light is dead and the filament is broken, or the glass is so black that you can't even see the filament, the only remedy is a new bulb.

If the filament is not broken, it can only mean that the current is not flowing through the filament.

Very likely, the problem will be found right there in the bulb holder. With the bulb removed from the holder, you will see the contact points through which current is fed to the filament - two contacts for a double-filament bulb, and one for a single-filament bulb.

The area where the blobs of solder at the bottom of the bulb press on these contacts is subject to corrosion which causes increasing electrical resistance in the circuit.

Any sign of this - usually a whitish deposit - should be removed with fine sandpaper, then the area should be sprayed with a contact cleaner-corrosion inhibitor.

The current through the bulb has to flow back to the battery, of course, and the break in the circuit could equally well lie in the "earth return path", which is normally simply the metal bodywork of the car and wires attached to it.

One end of the filament in the bulb is usually connected to the metal cap of the bulb, and that, in turn, connects with the bodywork through the bulb holder. The little prongs on the side of the cap serve, not only to locate the bulb in the holder, but also to provide a return path for the current via the bulb holder.

What does the future hold for vehicle lighting systems?

In the short term the so-called high-intensity discharge headlights will become increasingly popular on new models.

In these an electric arc, instead of a glowing tungsten wire, produces the light, which has a distinct bluish tint when compared with tungsten filament headlights. They are commonly called "xenon headlights".

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