The series comparative method has been used to measure the change of thermal conductivity &Dgr;Kof a semiconductor in a magnetic fieldB. A trial of the method showed the relative change &Dgr;K/K0, whereK0is the zero‐field value, was equal to −10% for InSb at 530°K,B=8000 G. The experimental arrangement also allows determination of the dependence onBof the thermoelectric figure of meritZ.