Physics: Greater range of results for resistance in experiment for shorter wire - why?

Question

I did an experiment observing how length of wire affects resistance of wire. I used the ammeter and voltmeter method.
For shorter lengths of wire (10, 20cm) the range of results (I did 3 repeats) is quite a bit larger than for the longer lengths. Therefore, when calculating uncertainty in resistance the value is very large (I used the method of calculating uncertainty based on range for each voltage and current and then added these percentages). Why is there a big range for these short lengths? I there less... stability(?) with a shorter length? Is it connected to the fact that these shorter lengths began to heat up?

Answer 1

The problem is more likely the voltage drop across your ammeter and instrument leads, and depending on type, the resistance of your voltmeter, which is in parallel with your resistance wire. This is especially true if you use a set voltage supply.

What you should do is vary the voltage to obtain the same (small as possible!) current through both long and short resistances. The voltmeter should be connected at the ends of the resistance wires.

The ammeter also has a resistance, so when connected in series with a short wire, it will comprise a greater part of the overall resistance in circuit. The longer resistance wires tend to "drown" the ammeter resistance into negligence.

All of this means there will be inaccuracies when measuring a short length, which will be minimised when you measure a longer length.

You need to get rapid meter readings to avoid errors due to temperature, i.e. you read the meters immediately after power connection, with the wires cold.

Answer 2

You don't say what the wire is composed of but I think that you are right in your last sentence, the shorter the wire, the more current will pass through it (depending on the impedance of your power supply) if the wire becomes warm then it will most likely increase in resistance (some materials decrease in resistance, some have to get very hot first though). Any air currents will cause variations in the temperature of the wire, the hotter the wire the more affected it will be by air movement. There are methods of measuring wind speed using this effect.

Answer 3

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