Iona Prep Physics Lab

Resistance of a Light Bulb


You will be using a Direct Current (DC) power supply and DC meters.  When using DC meters, the following rules must be observed:

  1. A voltmeter is always wired in PARALLEL and polarity must be observed.
  2. An ammeter is always wired in SERIES and polarity must be observed.

Your voltmeter has several scales. Connect to the 5 Volt scale.
You will use a Digital Multimeter as an ammeter. Use the 200 mA scale. (200 mA = 0.200 A).


In this procedure, keep the voltage <5.0 Volts DC or you risk damaging one or more of the meters.



  1. Draw a schematic diagram indicating the power supply, voltmeter, ammeter, and bulb. Schematic symbols
  2. Build the circuit according to your diagram.
  3. Have your instructor inspect the circuit before you plug in the power supply.
  4. Turn the power supply variable voltage to its lowest setting before you plug in the power supply.
  5. Beginning at the minimum voltage, increase the voltage and record voltage and current for at least 8 different values.  Do not exceed 5.0 volts.
  6. Record your data in a table like the following:


Voltage (Volts)

Current (Amperes)

Resistance (ohms)
















  1. Construct a graph with Voltage along the X axis and Resistance along the Y axis.
  2. From your graph, determine how the resistance of the bulb depends upon the voltage applied.


Conclusion: As the voltage increases, the resistance of the bulb (increases, decreases, remains constant, varies irregularly).