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Investigation into the resistance of a filament lamp.

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Investigation into the resistance of a filament lamp


To investigate Characteristics of a filament lamp

Background Knowledge

Filament bulb

The tungsten filament in a modern light bulb is supported by several molybdenum wires. The ends of the support wires are imbedded in a glass button at the top of the glass support rod. The copper and nickel lead-in wires, which carry the current to the filament, are supported by a glass support stem. One lead-in wire is soldered to the metal contact at the base of the bulb, while the other is electrically connected to the side socket contact. The contacts are separated by an insulating plate. An electrical current can pass in either direction through the filament.image00.png

The filament of a light bulb is a resistor. When a current passes through the resistor electrical energy is converted into heat. A light bulb converts current into power by the resistive element inside the light bulb. When the power level is sufficient to raise the temperature to above 1000 degrees Kelvin, visible light is produced, which can help us to estimate the temperature of the filament.

The length of time that you leave the light bulb on determines the total amount of energy that has been used. The hot filament then radiates a fraction of this energy in the form of light.

The current through the light bulb is notproportional to the voltage across it. This is because the current through the filament causes it to heat up, as the temperature of the filament changes the resistance also changes. The filament is therefore a non-ohmic device. We can use this change in the resistance of the filament to measure the temperature of the tungsten wire.

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I will be estimating the temperature of the filament lamp for different potential difference. The units for temperature will be Kelvins, for voltage it would be Voltage.


  • Voltage
  • Current of the filament
  • Temperature
  • Resistance
  • Power

Control Variables:

  • Length of the filament
  • Material type of the filament
  • Cross-section area of the filament
  • Current of the power supply

Safety precaution:

  • Electricity is dangerous so therefore be careful while connecting the circuit by having the power supply turned off at start.
  • Make sure that the current is not too high, which will cause the filament bulb to blow up and that is not safe working.

All typical safety rules for lab work should be followed. These include no running whilst experiments are in progress, hair tied back, jewellery tucked away and carrying out all experiments on benches clear of school books and standing up with stools tucked under benches. Also, as the potential difference is increased the more the current flows through the circuit. As this happens, it is extremely likely that the apparatus shall become hot, so particular care should be taken whilst handling it.

Fair test:

In order to make the results as accurate as possible, each reading should be repeated at least three times, and the mean of these three should be taken as the final reading. This would increase accuracy, by repeating the experiment we will remove any anomalous result.

The same pieces of apparatus should be used for each repeat, as even a slight variation in a piece of the apparatus. To make this experiment fair, we need to control the control variables.  

Circuit diagram:





  • Ammeter
  • Voltmeter
  • Variable Resistor
  • Filament lamp( 12V)
  • Connecting Wires
  • Crocodile clips( 2- to 3 to connect the
  • Thermometer
  • Power supply (D.C supply 12V)

Preliminary Measurements:

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2,250° C



2,375° C



2,500° C



2,625° C



2,750° C



2,875° C



3,000 ° C


Analysis of Results:

The graph is showing the relationship between the average voltage and the current in a filament lamp produces a curve. The curve begins very slightly at first and then gradually becomes steeper. It shows that as the voltage was increased, the current that was flowing through the wire also increased. There were some anomalous results, the reason for these result could be random errors where mine reading and recording of the results were wrong.


If you look at the colour of the filament at various points you should notice that the colour changes from red to shinny white. This indicates that the temperature of the filament lamp is increasing at every point. From my re-search we know that the mamimum temperature of the filament lamp is around 3,000 Kelvin, and at this temperature the colour of the filament lamp is shinny white. At 12V the colour of the filament lamp was shinny white which tell me that the maximum voltage of the filament lamp is around 12V. The room temperature was 23 Celsius, to convert this into Kelvin we add 273 to it. So the room temperature in Kelvin was 296; this was also the temperature of the filament lamp at 0volts.

Remember this is an estimate not an accurate answer, if you use the graph we could find the temperature at various points. For example if the want to know the temperature of the filament lamp at 6.50Volts, to do this you have to use the I/V characteristic graph where you read 6.5 volts horizontally and go up. Where 6.5 volts touch curve you read of the temperature value from there. So the temperature at 6.5 Volts would be 1500 Kelvin.

If the Voltage of the filament lamp was

In my re-search I showed 3 different way to measure the temperature I will use Stephan-Boltzmann formula to

As I said in my perdition that as the temperature of the filament increase so will the resistance increase?

...read more.

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