It is also generally the case (but not the criterion for distinguishing them) that the conductivities of metals at room temperature are higher than those of semiconductors. A solid insulator is a substance with a very low electrical conductivity. However, when that Conductivity can be measured, it is found to increase with temperature, like that of a semiconductor. For some purposes, therefore, it is possible to disregard the classification 'insulator" and to solids as either metals or semiconductors. Superconductors are a treat all special class of materials that have zero electrical resistance below a critical temperature