Advantages of four probe method

[ruchika]

how does contact resistance and sprearding resistance gets in eliminated in four probe technique for measuring resistivity in comparison to two probe technique where we encounter these resistances?

 

[f95toli]

It should be fairly obvious if you draw a circuit diagram.
In the ideal case (perfect current source and voltmeter with inifinite input impedance) the series/contact resistance is eliminated simply because the current is the same in the whole circui (no current flows into the voltmeter) and the voltmeter is only measuring across the sample.
Since you know the current and the voltage you then calculate the resistance from Ohms law

Note that there are of course instances where non-ideal effects come into playl; e.g..when measuring samples with very high (several megaohms) or low resistance (milliohms); so it is important to understand how the technique actually works.

Edit: I just realized that I assumed you were referring to "through" measurements (e.g. a resistor), but since you are referring to resistivity you might actually be asking about van der Paw measurements (of e.g. thin films)?

 

[sp105]

May I come to know that how the current remains constant throughout the resistance.

It should be fairly obvious if you draw a circuit diagram.
In the ideal case (perfect current source and voltmeter with inifinite input impedance) the series/contact resistance is eliminated simply because the current is the same in the whole circui (no current flows into the voltmeter) and the voltmeter is only measuring across the sample.
Since you know the current and the voltage you then calculate the resistance from Ohms law

Note that there are of course instances where non-ideal effects come into playl; e.g..when measuring samples with very high (several megaohms) or low resistance (milliohms); so it is important to understand how the technique actually works.

Edit: I just realized that I assumed you were referring to "through" measurements (e.g. a resistor), but since you are referring to resistivity you might actually be asking about van der Paw measurements (of e.g. thin films)?

 

[f95toli]

Well, the current MUST be the same as long as all the components are connected in series, right? That's one of the basic laws of circuit theory; current in=current out.
Hooking up the voltmeter won't change the way the current flows in the circuit, since we can assume that its input impedance is infinite (i.e no current is flowing in the leads going to the voltmeter).

 

[sp105]

thanks for such a kind reply

 

[goku1991]

query regarding four probe method

In four probe method, do we 'have' to mearure the potential drop between the center 2 terminals, or, could we swap the functions of the probes...
Also, can we, (theoreticaly), alter the distances between the probes which measure voltage, instead of all the probes being equidistant, that is...?

 

[Studiot]

Don't know if you are referring to the Wenner array for resistivity measurement in layered materials

http://www.google.co.uk/#sclient=ps...q=2&aqi=g5&aql=&oq=&pbx=1&fp=30b0f135803d42c7

or the series method described by others?

 

[goku1991]

@ Studiot:
I would be referring to the series method... thanks!