In determining thermal conductivities of solids not very good conductors of heat, the method least open to objection on theoretical grounds is the one in which a spherical shell of the substance to be tested is filled with, and the exterior surrounded by, some good conductor of heat, the temperatures of the conductors, inside and out, being observed by means of thermometers or thermo-junctions, and maintained constant by heat supplied, e. g. , electrically, to the inner conductor at a measured rate (fig. 1). Difficulties, mainly of a mechanical kind, present themselves, however, in the carrying out of this method, which render it advisable to sacrifice some of the theoretical simplicity, in order to make the method more practicable. These difficulties are overcome most easily by having the material to be tested, the conductor to which heat is supplied, and the outside conductors, in the form of flat circular discs of the same diameter (fig. 2), the good-conducting disc, C , to which heat 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-junctions 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.