You beat me to it Ted. The theory below is illogical.
>There has been some conversation here about current through lamps increasing
>as the voltage decreases because the lamp is rated at a particular wattage
>and Ohm's law was used (abused) to show that this was true. However, that
>isn't quite the way it works.
If the current were to increase as voltage decreased headlights would
be their brightest when the battery was dead, and buried. And conversely.
>The wattage rating on a lamp is an indication of the maximum wattage it will
>deal with without releasing excess smoke. A lamp can be thought of as a
>constant resistance device. That means that no matter how much smoke you
>force through it, the resistance remains essentially the same. It will, in
>fact, change as the filament heats up but that is inconsequential here.
Yes, the resistance (of the headlight) will change as the filament heats
up and when the filament burns out the headlight will have infinite
resistance.
>Wattage is calculated by Volts X Amps. Since the lamp is a fixed
>resistance, you find out how much current will flow through it by respecting
>Ohm's law V= I X R or I = V/R. Therefore, as the voltage drops so does the
>current. As they both drop, so does the wattage. If the voltage increases
>so does the current. If you increase the voltage too much, you will have to
>much wattage and exceed the rating for the bulb and the filament will break
>and spill smoke all over the inside of the bulb which will keep any more
>light from coming out of it.
Then the headlights will have that smoked-glass effect one sees on car
windows.
Mike
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