This paper is concerned with the experimental determination of the arrangement of the double‐plane probes and of an upper limit of a magnetic field for valid double‐probe measurement. One of the most reliable arrangements is that of two probes facing each other and situated parallel to the field. Also, the spacing of the probes must be significantly larger than 2&lgr;D. The upper limit of the magnetic field, below which the double‐probe method remains reliable, is determined mainly by the diffusion‐controlled collecting mechanism.