The volume resistivity of the samples, immersed in a digital-controlled oil bath to avoid oxidation, was measured with a high resistivity meter equipped with a DC voltage source. The applied electric intensity was 50 V/cm. In order to provide stable values of sample resistivity, silver paste was coated onto the surface of the sample to insure good electrical contact and the samples were put into the bath set to constant temperature. The variation of electrical resistivity over annealing time was recorded by a computer.