On Mon, Jul 6, 2009 at 1:27 AM, Sean Breheny<shb7@cornell.edu> wrote:
> You have a sharp eye, Solarwind :) You caught the inconsistency in my
> explanation.
>
> In the example I gave, one could say that grip pressure and sweat
> change the resistance, but that the current to voltage relationship
> would still be linear.
>
> That might be true, I don't know enough about human skin.
>
> However, there ARE examples of materials and devices which do NOT
> follow Ohm's law.
>
> Diodes, for example. Their voltage drop is approximately the natural
> log of the current.
>
> Another example is a salt water solution with conductive electrodes
> stuck into it. I don't know what the voltage to current relation is
> here but I suspect that it is very nonlinear and, therefore,
> non-Ohmic.
>
> A spark gap is another. When a spark gap is NOT arcing, you can
> increase the voltage until it reaches the breakdown voltage (depends
> on the gap/electrode geometry, the gas they are in, presence of
> ionizing radiation, etc.). Up til this point, the current will be
> zero. Then, suddenly, the arc starts and the current begins to rise.
> As the current rises, the voltage actually DROPS. If you attach a
> current source to the arc, and you turn up the current, you will see
> the voltage go DOWN, instead of up as you would expect with an Ohmic
> material.
>
> I would suspect that skin has an Ohmic region (i.e., up to a certain
> voltage it will be mostly Ohmic), but that it behaves in a non-Ohmic
> fashion at higher voltages.
>
> Sean

This is definitely something I should read up on. I thought Ohm's law
was universal.
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