resistors in series not acting ideal?

Variations in the bulbs and the Heisenberg principle.

The easiest explanation is that the resistance of the lightbulb changes as the current changes.

The way I would check this hypothesis is to put one lightbulb in series with a resistor and then measure the current through the circuit as you vary the voltage.

Take a couple of datapoints and then it should be easy to tell if the resistance of the lightbulb is changing.

Please let me know what happens.

the resistances of the bulbs were individually measured beforehand, so individual variations in the resistors is not a valid point. you cannot chalk up 40% error to Heisenburg on this...

ebrown: resistance changing with voltage...that is an interesting idea, I think this would imply that it is acting sort of like a diode...if a diode is formed by a substrate divided into an n-type region and a p-type region...it seems logical to me that this kind of performance could be achieved simply by having two conductors with different conductivity in series....i suppose that the filament and the contacts might have different conductivities so perhaps that is the explanation.

Ill do some more tests later..

I doubt this has anything to do with the mechanism that causes these effects in semiconductor (assuming these are normal incandescent lightbulbs).

Resistance does often vary with temperature. If the tests with a resistor and one lightbulb show that does vary with current I would guess that it is probably because the filament is heating up.

As a second test for this, you could try cooling down the lightbulb while it is in the circult.... or you could measure the resistance (out of circuit) if you had a convenient way to heat it up.

I like your rheostat idea better than cooling the bulb... unless you cool it evenly you risk shattering the bulb.