Measuring the resistance of a light bulb

Depending upon your available equipment, there are a number of methods available.

All of the methods, though, rely on measuring a number of different voltage and current values, and then using the relationship V = IR to calculate the resistance.

Method 1
<jpg here of digital power supply circuit>

1. Set up the circuit as shown.
2. Turn on the digital power supply.
3. Increase the power supply from 0V to 12V in steps of 1V, at each increment measuring both the voltage across the bulb (in volts) and the current flowing through the bulb (in amps).
4. Repeat step 3 two more times.
5. Calculate an average current and an average voltage for each increment.
6. Use V = IR to calculate the resistance for each increment.
7. Plot your results on a graph, with current as the independent variable (horizontal axis) and the resistance as the dependent variable (vertical axis).

Method 2

(Remember - these are non-standard circuit symbols.)

1. Set up the circuit as shown.
2. Turn on the variable power supply.
3. Increase the power supply through its range of outputs, at each increment measuring both the voltage across the bulb (in volts) and the current flowing through the bulb (in amps).
4. Repeat step 3 two more times.
5. Calculate an average current and an average voltage for each increment.
6. Use V = IR to calculate the resistance for each increment.
7. Plot your results on a graph, with current as the independent variable (horizontal axis) and the resistance as the dependent variable (vertical axis).

Method 3 - this method does not allow for the use of averaging as a mathematical method.
<jpg here of potential divider circuit>

1. Set up the circuit as shown.
2. Turn on the power supply.
3. Increase the resistance of the potentiometer/rheostat in gradual stages, at each stage measuring both the voltage across the bulb (in volts) and the current flowing through the bulb (in amps).
4. Use V = IR to calculate the resistance for row of your table.
5. Plot your results on a graph, with current as the independent variable (horizontal axis) and the resistance as the dependent variable (vertical axis).