Re: lightbulbs

[brian whatcott <inet@INTELLISYS.NET>]

At 17:34 11/9/99 -0500, Yvon Jean wrote:
> Here's a simple experiment I'm having trouble with.
> .. find the relationship between the resistance of
> the bulb and the illumination produced by the bulb.
>
> I tried 3 bulbs (1.5 V, 3.8 V, 6V) sequentially
> wired to the same  1.5 V drycell.
> The least light is obtained with the 6 V bulb and
> the most light with the 1.5 V bulb...
> The current and resistance values were obtained
> with a digital multimeter.  Here are the results:
> 1.5 V bulb        3.8 V bulb        6 V bulb
> 1.6 ohms        2.9 ohms            1 ohm
> 0.22 A            0.18 A                0.28 A

You measured currents with the lamps at various temperatures.
You measured resistance of cold filaments.

You mentioned the rated voltage of each bulb. This would produce a
white hot filament temperature giving a resistance about four times the
value of the cold filament resistance - of your 1.5 volt bulb for
 instance.  This lets you roughly estimate the nominal power of the
other two:
1st bulb actual power = 1/3 watt
2nd bulb rated power = 3.8^2/(2.9*4.25) = 1 watt (estimated)
3rd bulb rated power = 6^2/(1*4.25) = 8 watts.   (estimated)

 Bottom line - it's not simple to estimate the brightness
of bulbs of unknown power of known working voltage from their
 current across a known voltage.


brian whatcott <inet@intellisys.net>
Altus OK