# Light Bulb Resistance

If the voltage and current of a circuit containing a light bulb are measured using a multimeter after consecutively lowering the voltage (building of a data set), what (type of) function will describe the trendline of the data when graphed (voltage as a function of current)? Is there a limit to the function or would it be theoretically exponential?

The goal of this is to find the resistance (graphically) of the bulb; which I do know will not be a constant value as the filament heats up. Is it good enough to assume a linear relationship and therefore use Ohm's Law, V=IR, as the basis for finding the resistance? Or is there a better way to do this?

• In a tungsten filament bulb, the resistance goes up with temperature, in a carbon filament bulb it goes down. Dec 30 '19 at 18:58

Tungsten has a pretty linear relationship between temperature and resistivity in the range being considered here.

So the resistance of the bulb will be quite linear with the temperature.

But the temperature is a complicated function of the electrical heating power, the environmental cooling, and the history: you can find different temperatures hence resistances depending on whether you’re raising or lowering the voltage.

Given all that anyway, what does R as a function of V look like? At low voltage it starts at some constant value, then starts to rise linearly. This ends up creating a (mostly) constant current at the voltage rises: 