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The resistivity of Constantan

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Carol Slack

The resistivity of Constantan

The resistance of a piece of wire is dependant on its length, cross-sectional area and the type of metal the wire is made of.  The resistance of a given wire can be calculated using the following equation:

                                             R = ρ L / A                        where:    L = Length (m)

A = cross-sectional area (m²)

ρ = resistivity of the metal

By rearranging the equation the resistivity of the metal can be calculated:

                                          ρ =  R A / L

The resistivity differs depending on the metal however it is constant at room temperature for each metal.  This means that two pieces of wire made of the same metal and at room temperature should give the same result when calculating resistivity regardless of its length and cross-sectional area.  The following equation can be used to calculate the resistance of a wire:

                                                R = V / I      where:  V = volts

                                                                                   I = amps

                                                                                  R = resistance

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In the interest of safety all electrical appliances should be regularly checked.  It is also important that the wire is not handled while in use in the experiment as it may heat up.  To ensure little error the crocodile clips should be at the very end of the wire as the length is a factor in the calculation.  If the wire curls onto itself and is touching at any point then this will affect the results as it

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The equipment used was limited as to how accurate the readings could be with the accuracy being ± 0.01 for the ammeter and only ± 0.1 for the voltmeter.  This may have affected the results and could account for why the graph for the 34 S.W.G. wire did not form a straight line and so a line of best fit had to be drawn.  The graph for the 30 S.W.G. did form a straight line as expected so perhaps there was less error in the testing on this wire.  If the experiment was to be repeated, a more accurate voltmeter and ammeter should ideally be used.  It may also be beneficial to use wires that come from the same batch and company to ensure this is not a factor affecting the results.


Kaye and Laby (1995) Physical and Chemical Constants 16th Ed : Longman

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