The resistance of things like lamps and glow plugs is not constant. If you
measure the resistance of a lamp when the filament is cold, it will be far less
than the resistannce of the same lamp at normal operating conditions (as
calculated from V/I ). That's why lamps draw a high inrush current when switched
on. You can verify this with a small bulb and an ohmmeter. The meter reading
will chsange with time as the bulb filament heats up, due to the meter current.
You cannot accurately predict with simple arithmetic what the current, power
consumption or light output of a bulb will be when operated at other than it's
nominal ratings. All are power laws, of the form (V/Vnom) ^ N, where N is some
number. As I recall, N is 5 for the brightness of a lamp, but I'm not certain.
Also, the color temperature of a lamp varies with operating voltage, and this
factors into the percieved brightness. In WW II, the army used lamps operating
at reduced voltage, because that shifted the light output more into the red, as
illuminators for infra-red sniperscopes.
-John
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