Semiconductors that show current instabilities usually have an associated negative conductivity for a certain range of electric field. Information about this region may be obtained from direct two-terminal measurements on the material. A direct method of measurement is described in which a voltage pulse, in the form of a fast-rising ramp, is applied to the sample and the current waveform is simultaneously observed. Also described are techniques for generating suitable driving waveforms, and for measuring and separating the conduction and displacement current components. The method has been used for measuring the characteristics of materials with values of negative dielectric relaxation time in the -50-picosecond region. Copyright © 1969 by The Institute of Electrical and Electronics Engineers, Inc.