Abstract A knowledge of thermal conductivities has now become of importance in a variety of industries; for example, in the designing of electrical machinery and in the lagging of refrigerating and high-temperature plants. While the literature on thermal conductivity is fairly extensive, little attention has hitherto been paid to the design of apparatus for accelerated precision tests. For example, the ingenious methods of experiment devised by Prof. Lees in the course of his classical and extensive investigations, while admirably adapted for the particular object he had in view, would be somewhat laborious if the number of specimens to be studied were considerable. Thermal conductivity is a physical constant not capable of ready determination by simple appliances, and in the design of the apparatus described later our aim has been to produce an equipment which will give the desired data with the minimum expenditure of time and labour.