The twin transient-state cylindrical-probe method was investigated to determine the thermal conductivity of soil. The apparatus consists of the same two probes, which are made of stainless steel pipe, into which heating wire and a thermocouple are placed. One probe is introduced into the standard material (1 percent agar solution) and the other into the measuring material. An electric current is passed through the heating wires of both probes for about 40 sec. Changes in the temperature of the probes are recorded with an X-Y recorder. The locus of the X-Y recorder is a straight line, and the tangent of the line is the ratio of the thermal conductivity of the two materials. The thermal conductivity of soil can be measured directly from the tangent with a high accuracy.
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Tatsuaki Kasubuchi (1977) studied this question.