Apparatus has been designed and constructed capable of measuring the photo‐Hall effect for mobilities as low as 0.1 cm2/V‐sec, in photoconductors with resistivity as high as 1010&OHgr;‐cm, over the temperature range from 77 to 450°K. Temperature control of the photoconductor crystal by thermal conductivity through gaseous helium eliminates many of the problems associated with temperature control by direct thermal contact to the crystal. Voltage signals down to 100 &mgr;V are detected by vibrating reed electrometers, using five contacts to the crystal.