or dc voltage pulses in which case the voltage curve has the appearance of a square zig zag represented in fig4 b in each of these two cases the period t 2 of the interruptions is advantageously chosen to be of the same order of magnitude as the diffusion time of the particles over one pitch of the array of striae in the case of temporal modulation of the amplitude of the electric field the generator 6 delivers a modulated voltage for example a sinusoidal wave as represented in fig4 c the frequency of this sinusoidal wave being in certain cases advantageously chosen to be of the order of magnitude of the inverse of the dielectric relaxation time of the particles to be separated or entrained and the pitch of the array of striae then being chosen such that the diffusion time for one particle over this pitch is of the order of magnitude of the said dielectric relaxation time in the case of adding a sensitizing field it is possible for example to use as diagrammatically represented in fig5 which shows also diagrammatically but more fully than fig1 an embodiment of the invention irradiation by sensitizing light radiation coming from a lamp 18 in this case the lamp 18 is illuminated so as to irradiate the separation zone and the generator 6 delivers a ac or dc voltage without more particular modulation for example as represented in.