A method of measuring the sheet resistance of slices of semiconductor. Four equally spaced probes are used – the two outer probes supply a small current I and the two inner probes are connected to a high-impedance voltmeter to measure the potential difference V developed. In the ideal case of a homogeneous semi-infinite material the sheet resistance R_s is given by

where s is the spacing between the probes. In practice, the material is not infinitely thick; the conductive layer is usually very thin relative to the probe spacing, and a correction factor of π/log_e2 must be applied. The lateral extent of a slice is not infinite and a second correction factor must be used – the value of this factor is a function of the slice size and positioning of the probe.