Direct measurement of the resistivity weighting function

We have directly measured the resistivity weighting function - the sensitiv ity of a four-wire resistance measurement to local variations in resistivit y - for a square specimen of photoconducting material. This was achieved by optically perturbing the local resistivity of the specimen while measuring the effect of this perturbation on its four-wire resistance. The weighting function we measure for a square geometry with electrical leads at its cor ners agrees well with calculated results, displaying two symmetric regions of negative weighting which disappear when van der Pauw averaging is perfor med. (C) 1998 American Institute of Physics. [S0034-6748(98)03412-1].