Understanding wire resistance

[Rifrug]

Hi guys, I would like to know if I take 100 meter coil of gp wire, and I measure the resistance over the two ends. Than I roll the wire out and measure the resistance again, will the resistance still be the same?

[Lourens.dL]

I suppose while using DC as with a multi meter to measure with it should be the same.

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[Dave A]

The trouble with coils is you get inductive resistance.

[Lourens.dL]

The trouble with coils is you get inductive resistance.

I was thinking about that but how relevant is it with DC? I can't remember the theory. [emoji23]

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[Rifrug]

The trouble with coils is you get inductive resistance.

The reason I asked this question is when doing a earth resistance test, I have a reel with about 100 mtr wire on it to test from main earth to say a motor that is 80 mtr away. I measure the resistance of the 80 mtr wire, than subtract it from my actual reading at motor. But it seems my readings can sometimes be inconsistent when I don't roll out the whole reel before taking readings. Could this be possible? I use a meggar but on 3ohm continuity scale that I also zero to make things easier.

[Justloadit]

The coiled wire could be acting as an antennae and a voltage is being induced in the coil from the stray voltage in the air, especially if there are large current loads in the vicinity. Undoing the coil reduces the amount of stray voltage induced in the wire. This could be the reason that you get fluctuating readings.

[Dave A]

I was thinking about that but how relevant is it with DC? I can't remember the theory. [emoji23]

Any change in current will have an inductive effect. The classic example is the old fashioned car ignition coil which is a DC system - the big voltage spike on the secondary side was when the points broke the connection (sudden drop in current on the primary) rather than when the contact was made.

Then as Justloadit covers above, the coil could act as a secondary to any stray magnetic fields in the vicinity.

[Rifrug]

Thanks guys it makes more sense now because the fluctuating readings were normally when the wire is not rolled out all the way, I did not understand the reason for this.
I will now make sure I roll out my earth reel all the way before doing earth resistance tests.
I like learning something new everyday.
Thanks a lot