An apparatus is described for measuring the Hall effect of low mobility samples at high temperatures. The measuring method is of the double ac type. An alternating current at 510 cps, in conjunction with a low frequency magnetic field at about 2 cps, is used. The use of single or double phase sensitive detection is discussed. A temperature controller with a stability of about 2×10−2C° at 1000°C and a sample holder assembly are described. Results of measurements on NiO samples are given, indicating a noise level equivalent to mobilities of 3×10−4cm2/V sec.