This paper introduces some preliminary consideration on non uniform resistance changes in silicon square photoconductors resulting when the surface exploration is made by small spot laser light. These results seem to be similar to those obtained from a chemically sensitive material such as, for instance SnO2 left in air at a suitable temperature, for which the resistance decrease in presence of CO. A simple theory is considered for the determination of the approximated overall resistance value of a thermistor (or chemically sensitive thermistor) on the basis of sensitive sites localization.