The electrical resistivity is obtained measuring the minimum value of the electrical resistance of a dust layer deposited between two electrodes when cc voltages of 110 V, 220 V, 300 V, 500 V and 1000 V are applied. With the object of classification, when the electrical resistivity of a dust is less or equal than 1000 Ω.m the material is considered as conductive. Otherwise, that is to say, when the electrical resistivity is higher than 1000 Ω.m, the material is classified as non-conduThe electrical resistivity is obtained measuring the minimum value of the electrical resistance of a dust layer deposited between two electrodes when cc voltages of 110 V, 220 V, 300 V, 500 V and 1000 V are applied. With the object of classification, when the electrical resistivity of a dust is less or equal than 1000 ?.m the material is considered as conductive. Otherwise, that is to say, when the electrical resistivity is higher than 1000 Ω.m, the material is classified as non-conductive