400 DR. C. H. LEES ON THE THERMAL CONDUCTIVITIES OP SOLIDS 



is supplied, being placed between two discs of the material to be tested, A and B, and 

 two other good- conducting discs, D and E, placed in contact with the outer surfaces. 

 By placing thermo-j unctions in each good-conducting disc we can determine the 

 relation between the heat supplied to the inner disc and the differences of temperature 

 between the inner and outer discs. 



If all the heat supplied to the inner disc flowed through the substance to be tested 

 to the outer discs, the thermal conductivity of the material would be thus deter- 

 mined, but some heat is lost from the curved surfaces of the discs by conduction and 

 radiation through the air to the enclosure in which the discs are placed. The amount 

 thus lost can be determined by observing the temperature of the enclosure, if the loss 

 per square centimetre per degree excess of temperature of the edges of the discs over 

 the inside of the enclosure were known. 



To enable this quantity to be determined directly by experiment, the whole of the 

 hea,t supplied to the discs must be lost in the same way, i.e., by conduction and 

 radiation through air, and to satisfy this condition, the collection of discs should be 

 placed in the centre of an air bath kept at constant temperature (fig. 3). The heat 

 lost from the curved surfaces of the discs then follows with sufficient closeness the 

 same law as that lost from the flat surfaces of the outer discs, and the relative 

 amounts'of the two can be determined from a knowledge of the areas and temperatures 

 of the various surfaces, if the surfaces have the same "outer conductivity" or 

 " emissivity," an equality which is easily secured by varnishing them. 



Once the method of surrounding the discs by an enclosure at constant temperature 

 has been adopted, a further simplification of the arrangement of discs is possible. If 

 one of the discs of the substance under test and one of the outer discs were removed, 

 the relative amounts of heat lost by the heating disc, C (fig. 4), by conduction and 

 radiation directly to the air from its exposed, surfaces, and by conduction through the 

 disc, B, of material experimented on, partly to the air directly by conduction and 

 radiation from the surface of B, and partly by conduction to the outer good-conducting 

 disc, E, arid radiation from its surfaces, could still be calculated, and, the total heat 

 supplied being known, the conductivity of the material determined. On account of 

 this method requiring only one disc of a substance the conductivity of which is to be 

 determined, it has been adopted in the experiments to be described. 



Description of Apparatus. 



The materials to be tested were cut into discs of 4 centims. diameter, and of 

 various thicknesses, in order to make the magnitude of the difference of temperature 

 of the two flat surfaces, suitable for observation with the thermo-j unctions employed. 

 The discs between which the material was placed were of copper, and had the same 

 diameter. On one side of the substance to be tested was placed a single copper 

 disc, M (fig. 5), -320 centims. thick, on the other side a compound disc, consisting of 



