This manually operated four point probe instrument measure the average resistance of a thin layer or sheet by passing current through the outside two points of the probe and measuring the voltage across the inside two points. These measured values can be used to determine the sheet resistivity as well as bulk resistivity of the sample.
If the spacing between the probe points is constant, and the conducting film thickness is less than 40% of the spacing, and the edges of the film are more than 4 times the spacing distance from the measurement point, the average resistance of the film or the sheet resistance is given by:
Rs = 4.53 x V/I
The thickness of the film (in cm) and its resistivity (in ohm cm) are related to Rs by:
Rs = resistivity/thickness
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