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An investigation to find out what factors affect the resistance of constantan wire.

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Catherine Lynskey

An investigation to find out what factors affect the resistance of constantan wire.


Ohm’s law and Resistance

In 1826, Georg Ohm discovered if the temperature of a wire remains constant, the current flowing through a metal wire is proportional to the potential different across it. (If you double the p.d the current is doubled.) Ohm’s Law also states the ratio of the voltage across a conductor to the current flowing through it is constant, the resistance. The resistance of a conductor depends on its length, thickness and the material it’s made from. The unit of resistance is ohm.

This was then named Ohms Law.

Resistance is when the wire tends to resist the movement of electrons. It has a certain resistance to the current. The greater the resistance, the more voltage is needed to push a current through the wire.

Resistance is calculated by:


I predict that the longer the wire is the greater the resistance; this will also be the same for the wire with a small cross sectional area.

...read more.


As cross-sectional area increases, the resistance decreases.As temperature increases, the resistance of the wire increases.


The apparatus that will be used for the preliminary experiment will be:

  • Battery.
  • Variable resistor.
  • Amp metre.
  • Volt metre.
  • Wires to connect the circuit together.
  • Constantan wire.


During this experiment I am going to measure the amps and the volts of selected wires and their cross sectional area. I will then work out the resistance of the certain constantan wire.

The measurements that I will be using are;

  • Diameter- 0.08
  • Diameter- 0.71

Using both of these pieces of wire, separately, I will attach them to the circuit as shown on the previous page. I will then measure the wire to 50cm and take eight readings from it using a variable resistor and then again I will do the same but with 1 metre of the wire. I will do this with both of the diameters.

I will then construct my results on a graph. As this is the preliminary I will not do repeats on the anomalies but for the final experiment I will redo anomalies.

Method of preliminary experiment

  • I will first connect the circuit together.
  • Then I will connect the wire with the diameter of 0.08 and a length of 50cm.
  • I will turn the power supply on using a voltage of 2.
  • I will take eight readings from the volt metre and eight reading from the amp metre, using a variable resistor and then I will do the same with 1 metre.
  • I will repeat this using the wire with a diameter of 0.71. Again I will do 50cm and then 1 metre.
...read more.


  • 25cm.
  • 50cm.
  • 75cm.
  • 1 metre.


Through my final experiment I have proved my hypothesis. The experiment supported my hypothesis. As the length doubles so does the resistance, also as the diameter is doubled, the resistance is decreased by half.

This is due to Ohm’s Law, which states the ratio of the voltage across a conductor to the current flowing through it I a constant, the resistance.

The temperature also remained the same throughout the experiment because the results were accurate and therefore it was a fair test because all of the experiments had the same amount of kinetic energy.

In the experiment with the diameter of 0.71, at 0.40 amps with 25cm of wire was 0.14 volts and at 0.40 amps with the length of 50cm, the volts were 0.30, this is nearly double, so my hypothesis is proved, as the length is doubled so will the resistance be doubled.

With the results that I had placed in tables, I constructed two graphs, one for each diameter. On each graph there was one anomaly, so I redid the anomalies so that I could get more accurate results and I constructed another graph to place the new results from the anomalies.

...read more.

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