The previous post describes my students' results showing that a flashlight bulb's resistance varies. Over the available voltage range of 2 V to 8 V, the resistance (determined by the slope of a voltage vs. current graph) varied from about 50 V to 80 V.
The question was, what does an ohmmeter read when placed directly on this bulb?
Consider how an ohmmeter generally works. It puts an awfully wee voltage across the bulb, and measures the resulting wee current through the bulb. Then the meter essentially uses ohm's law to calculate resistance. (That's why you have to disconnect the bulb from the battery in order to use the ohmmeter.)
In the context of our experimental voltage-vs.-current graph above, the ohmmeter is measuring an out-of-range data point, way off down and to the left of the portion shown. By extrapolating the curve shown, we could guess that we should get a shallower slope and thus a smaller measured resistance.
Sure enough, the meter measured about 8 ohms, a full order of magnitude less than the resistance in the bulb's operable range.
Again I caution teachers: this is a cool and somewhat unexpected result. Nevertheless, it's rather irrelevant to the typical practical analysis of a bulb. The bulb only glows at all with a volt or two across it; the bulb is only rated to about 6 V, meaning it is likely to burn out over that voltage. In the operable range, the resistance is reasonably steady. The resistance only drops by an order of magnitude when the voltage is dinky.
The next question: How can we experimentally extend this graph?
My variable DC supply only goes down to 2 V. I could get a 1.5 V battery to get one more data point, but that's all I can think of. Does anyone have a suggestion of a way to explore the parameter space below 1.5 V?