The resistivity measurements are made on the flat ends of the crystal by the four-point probe technique. The current I is passed through the outer probes and voltage is measured between the inner probes. If S is the probe spacing in centimeter, the measure resistance is converted to resistivity using the formula

• A. \(\left( {\frac{V}{I}} \right)S\)
• B. \(\left( {\frac{V}{I}} \right)2\pi S\)
• C. \(\left( {\frac{I}{V}} \right)2\pi S\)
• D. \(\left( {\frac{I}{V}} \right)2\pi \)