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I have currently been working with a sample that "appears to" decrease its resistance when I cover it and protect it from light.

Basically it presents the opposite behaviour of a photoresistor.

What kind of phenomenon may cause this? Or it has to be a measurement error?

EDIT: I am talking about typical room light from fluorescent lamps on the ceiling.

Experimental Setup: Sample: A dense, sintered, ceramic pellet (with a disc shape, 1 cm in diamater and ~3mm thick). The faces of the disc were carefully painted with conductive silver ink to make the electrical contacts. An Agilent 4294A impedance analyzer was used to measure the electrical properties with a 0.5 V AC signal. This was done at air, in a room with the ceiling lights turned on. Approaching my hands to the sample, blocking the light, the resistance increases.

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    $\begingroup$ Does your cover contact the sample? $\endgroup$
    – akhmeteli
    Commented Oct 20, 2013 at 21:42
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    $\begingroup$ what is your sample? resistor? piece of wire? $\endgroup$
    – Aksakal almost surely binary
    Commented Mar 3, 2014 at 20:38
  • $\begingroup$ @Aksakal It is a ceramic. The exact composition is not important for this question. $\endgroup$
    – cinico
    Commented Mar 5, 2014 at 3:15
  • $\begingroup$ If the silver ink patches are big enough can you connect both contacts to the same silver patch? I'm suspecting its something to do with silver ink $\endgroup$
    – Aksakal almost surely binary
    Commented Mar 7, 2014 at 12:43
  • $\begingroup$ @Aksakal Nope. The painting of the surfaces is carefully performed and we test the electrical contact before making the measurements. I can assure you that the surfaces are not in short-circuit. $\endgroup$
    – cinico
    Commented Mar 7, 2014 at 15:02

3 Answers 3

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As you measure impedance in AC it may be related to change in capacitance of the system when you came close with your hand. This principle is used for capacitive sensors of presence. Do you see the same effect when you turn off the lights rather than blocking it with you hand? You can just stay by the light switch and turn it on and off, without changing your position. So far you did not prove that is the effect of the light and not of your hand.

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It can be just the dependence of the material with temperature due to the heating from light.

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  • $\begingroup$ I'm sorry I didn't make it clear: I am talking about typical room light from the fluorecent lamps on the ceiling. That effect cannot justify such a big change. $\endgroup$
    – cinico
    Commented Aug 20, 2013 at 21:00
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We know that at lower temperatures conductors achieve superconductivity. So maybe your samples are heating in presense of light, and upon cooling in absense of light going towards superconductivity.

Although once when I had been working on building a circuit, I was experiencing similar results from my photoresistor. Alas it turned out to be some sort of error which fixed itself when I rebuilt the circuit, so I would suggest you to retry a few more times.

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  • $\begingroup$ Uh, he said nothing about superconductivity. $\endgroup$
    – YungHummmma
    Commented Mar 3, 2014 at 20:40

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