[ 495 ] 



LIV. On a Simple Apparatus for determining the Thermal 

 Conductivities of Cements and otlier Substances used in the 

 Arts. By Charles H. Lees, D.Sc, and J. D. Chorlton, 

 B.Sc, Joule Scholar of the Royal Society*. 



THE following method of determining the thermal con- 

 ductivities of bad conductors has been designed with 

 the object of simplifying as much as possible the apparatus 

 and observations required in making determinations which 

 are not required to be of the highest order of accuracy, but 

 in which errors of more than 2 per cent, are to be avoided. 

 When the constants of the apparatus have once been deter- 

 mined, the only observations necessary to determine a 

 conductivity are the thickness of a sheet of the substance 

 used and the temperatures of three thermometers. It is 

 hoped that this simplification will lead those who require bad 

 conductors of heat for structural purposes to carry out their 

 own tests of the materials they have available. 



Method. — The apparatus consists of a flat cylindrical metal 

 box, of 11*4 cms. diameter and 3 cms. depth, through which 

 steam . can be passed. The bottom of the box consists of 

 a circular brass plate 1*3 cm. thick in which a radial hole 

 reaching to the centre is bored. In this hole a thermometer 

 is placed with its bulb at the centre of the plate. The top 

 and sides of the box are covered with green baize to prevent 

 loss of heat as far as possible. This vessel is supported on 

 a circular plate of the material to be tested, which in its turn 

 is supported on a brass disk similar to the one forming the 

 base of the heating vessel, and like it provided with a radial 

 hole and thermometer. 



The lower disk is suspended horizontally from a support 

 by three strings attached to three short pegs projecting from 

 the edge of the disk, and hangs about 30 centims. above the 

 table. About 10 centims. above the table a thermometer is 

 placed horizontally, with its bulb under the centre of the 

 lower plate to give the temperature of the air ascending to 

 the disks. The bulb of the thermometer is protected against 

 radiation from the lower disk by a bright metal screen. 



The two surfaces of the disks which come into contact with 

 the material to be experimented on are amalgamated, so that 

 if the material is a solid, contact over its entire surface may 

 be obtained by using a thin mercury film between the solid 

 and disks. In order to determine the thickness of the 



* Communicated by the Authors. 



