4-wire (Kelvin) resistance measurement

[SteveChandler]

Wow that is really playing tricks with my mind. There are a total of four wires, two red ones and two black ones between the heatshrink?

[Darin.K]

If you want to understand what is going on, here is a simple picture.  The current leads generate a voltage between two points on the sample, and the voltage leads measure the potential between the two contact points.  In a good conductor, the current flow in the sample is predominantly along the line between the two current lead contact points.  Away from that line, little current flows and the potential is nearly constant.

Ideal:  You measure the potential difference between the two contact points of the current leads, largest possible potential difference, highest inferred resistance.

Clips Closed:  Close to ideal, measuring most of the potential drop.

Clips Open:  Now you measure a much smaller potential difference, this would lead you to infer a smaller resistance.

@Steve:  I would ignore the picture completely, it is a distraction.  A picture of the "actual" leads would probably be taken on top of a cluttered benchtop, this is a stock photo.  Even a two-wire measurement would show the same effect (it would just be tricky to account for the lead resistance).

[SteveChandler]

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@Darin.K wrote:
 

@Steve:  I would ignore the picture completely, it is a distraction.  A picture of the "actual" leads would probably be taken on top of a cluttered benchtop, this is a stock photo.  Even a two-wire measurement would show the same effect (it would just be tricky to account for the lead resistance).

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Ah. I thought it was the actual leads. I was kind of wondering how he got it to have a completely white background like that.

[Darin.K]

Of course now it sounds like a stock photo of the actual leads....  Anyway, it is clearly a four-wire measurement.

[Intaris]

There's an explanation HERE.

Shane.

[jcarmody]

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@Darin.K wrote:

Of course now it sounds like a stock photo of the actual leads....  Anyway, it is clearly a four-wire measurement.

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Bingo!

[jcarmody]

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Darin.K wrote: 

Clips Open:  Now you measure a much smaller potential difference, this would lead you to infer a smaller resistance.

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If I hadn't put this in BreakPoint I could have marked this as the solution.  It makes sense, anyway, as it matches my observations.

Thank you.

[Ben]

Request the moderator move this thread to the LV forum.

It will get more exposre, you can mark solution and it will show up in a search (cough cough hack hack) of that forum.

And if the situation is still the same as in the past, it will show-up on the various "mirror sites" and therefore help more people.

Ben

[Darin.K]

Actually we have been flogging a few students for posting non-LV engineering problems on the LV board so his feels like a good home. I could see the utility of a new board for technical problems to tap the collective knowledge without being buried in jokes or trespassing on other boards.

[SteveChandler]

Excellent idea! You should post that to the feedback forum.