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A View from the Bridge, acts 1 and 2.

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The formula for finding the resistivity of a wire is

        Where: R is resistance

                     P is the resistivity

                     A is the cross-sectional area

                     L is the length

The variables involved are the cross-sectional area (A), the length (L), the

Resistance ® and resistivity (P).  The resistivity of every wire is constant

So the changing variables are the resistance (r=v/I) where the resistance is

The dependent variable.   Keep the area constant.    

To calculate how the change in length affects the resistance, we plot a graph

Of length against resistance.  The resistance should be proportional to the

Length, R∝L (prediction) according to Ohms law i.e. there should be an

Increase in the resistance as the length increases- the longer the length, the

Greater the resistance.



Set up the circuit as shown in the diagram above, start the experiment by taping

The constantan wire of 100cm the has been measured to the metre rule.  Make

Sure the wire is carefully tightened at both ends of the metre rule then connects

...read more.


Piece of wire throughout the experiment.

Try to avoid parallax error by using a digital ammeter and voltmeter instead of a

Full-scale deflection ammeter and voltmeter.

Ensure that the wire is carefully tightened to both ends of the meter rule and avoid

Wires with loads of twists, which could alter the length and cross-sectional area

Of the wire.

To increase the decree of accuracy of the recorded result and plotted graph, take

Three sets of results and calculate the average, even when calculating the area.

Record the diameter at a minimal of four different points to be able to measure

The diameter of the wires accurately as possible.

Use terminal blocks instead of crocodile clips because the crocodile clips kept

Slipping out from each other and disturbing the circuit.  These terminal blocks

Make it easier to connect the circuit properly, but on the other hand, they make it

Difficult to measure the length accurately using a meter rule, so use larger length

So this inaccuracy is almost negligible.

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=44.2 – 0.40  = 4.02

  0.80 – 0.06     0.74

   = 5.43

The gradient = 5.43

AREA: πr where r = 0.23

πr  =  π(0.23x 10   )

     = 1.06 x 10

with the gradient ,m , I can now calculate the value for resistivity of constantan


R=ρL      ρ=RA

     A              L

 The formula for a straight line graph, y=mx and here,

ρ = R   A  


ρ= y, m= R/L, x = A

ρ = mA

   = 5.43x (1.060x 10)

   =5.75 x 10

CONCLUSION: At the end of my experiment, the resistivity of constantan wire

Is 5.75 x 10


The anomalies seen in my results (graph) were as a result of approximations. After I calculated the resistance, I approximated the result in order to be able to insert the numbers on my graph sheet and this affected my graph (gradient) because when

I plotted the graph without approximating; I got a perfectly straight-line graph.

Conclusion:  I feel my experiment (practical) went well and was suitable for the

Task given.  This is shown by the fact that I achieved consistency and proves that

This experiment can be both reliable and accurate. My result is reliable as they are consistence with each other and I have managed to refrain from making large errors.

Calculating the percentage error:  

=5.75 x 10  – 5.2 x 10   x  100

          5.2 x 10                     1

...read more.

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