The theory of cylindrical probes for measuring thermal conductivity is extended to the case of a probe of finite conductivity containing a line source at its centre. This provides a more realistic approximation to most actual probes than the theory for a probe of infinite conductivity developed by other authors. New experimental results are presented which are in complete agreement with theo!'Y' It is shown how an estimate can be obtained of the magnitude of a possible thermal contact

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... between the probe and the medium and how its influence on the measured conductivity can be assessed. Conditions under which the theory of the infinite line source can be applied with a sufficient degree of accuracy are treated. The properties of various probes described in the literature are reviewed in this respect. The importance of measuring both the heating and the cooling branch of the temperature against time curve is emphasized. Difficulties in measuring thermal diffusivity with-the probe are briefly discussed.