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I am using a simple wheatstone configuration to measure resistivity of a material I have.

enter image description here

The problem is that when I am measuring the Voltage (Vg) with a multimetre and it is oscillating(fluctuating). It doesn't let me have an accurate reading since multimetre shows different values change each second.

I checked my circuit by measuring a 10k,12k and 15k ohm resistances without any problem (no fluctuation in the reading), so the circuit is fine.

What do you thing the possible causes of this oscillation on the readings? material structure/composition? the way how I should measure the Vg voltage?

Pd: Vs is DC

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    \$\begingroup\$ A multimeter is a poor tool to see what's going on. Are your fluctuations Volts or millivolts? \$\endgroup\$ – Scott Seidman Aug 18 '16 at 15:43
  • \$\begingroup\$ It would be helpful if you provide more information. What are the values of R1, R2, R3 and Vs? What is the expected value of the material you are trying to measure? What multimeter are you using and on what mode and range? What do you mean by oscillation since real oscillations are unlikely with a DC source. \$\endgroup\$ – Barry Aug 18 '16 at 16:15
  • \$\begingroup\$ What's the material? could it be changing? warming up? What sort of resistance are we talking about? \$\endgroup\$ – George Herold Aug 18 '16 at 16:16
  • \$\begingroup\$ What is the frequency its oscillating at? Is it AC mains? If your measuring this with a DMM then why don't you just scrap the Wheatstone bridge and do a kelvin measurement? \$\endgroup\$ – Voltage Spike Aug 18 '16 at 19:08
  • \$\begingroup\$ Thanks for your comments. R1=R2=R3=10000 ohms. The material is a sheet (mixture of PDMS and Grapehene). There is no expected value since that is what I would like to figure out. DC is the input (5V). The thing is that I do not have any idea why it is oscillating (0.16 - 0.28 V from 0.16,0.17,0.18,....0.28 and then 0.28,0.27,0.26,.....,0.16). What I would like to know is the reason to have this oscillation. \$\endgroup\$ – Ivan Aug 18 '16 at 22:18
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Typical DMMs use reasonably high input impedances between 1-10MOhm but source values approaching this would make for slower settling times.

More likely the contact resistance is changing and swamps the surface resistance reading.

The use of 4 point probing may be indicated. https://en.wikipedia.org/wiki/Four-terminal_sensing

The Wheatstone bridge (invented for nulling measurements) is not often required in the modern day unless you are measuring small changes in bridge resistances like opposing changes in strain gauges and such. The resistance bridge works best when balanced so not ideal for automated measurement of unknown resistance values.

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Not really an answer, more in the realm of advice:

First, check your meter: fresh batteries, work the range switch several times and/or clean it, check conduction of the probe wires.

Also possible: your DC source is noisy. Batteries, for instance, can respond to atmospheric pressure or internal pressure (generate a triangle wave when they vent gas during discharge).

Next (you've probably already checked this) you can be dealing with a loose wire or bad insulation. Could your experimental apparatus have temperature or illumination sensitivity? Or maybe your samples are photoconductive? Could you be exhaling onto a film that absorbs vapors? Handling a sample that changes when warmed by the touch of a hand?

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