AND LIQUIDS AND THEIR VARIATION WITH TEMPERATURE. 419 



conductivity of which was to be determined, was poured on to the lower disc, where 

 it was retained by the raised edge. The three discs were then placed on the 

 horizontal top of a closed vessel through which a stream of cold water was kept 

 flowing, and they could, if necessary, have a cover placed over them. On sending a 

 current through the coil, (7, part of the heat generated flowed through the discs 

 to the top of the cold water vessel, the rest was lost by conduction and convection 

 in the air above the coil. After the distribution of temperature has become steady, 

 the heat flowing through the rubber from U to M proceeds either through the layer 

 of liquid to L, or is lost from the surfaces of M and of the liquid. As the tempera- 

 ture of the water flowing through the vessel underneath the discs was always a few 

 degrees less than that of the air of the room, it was possible, by regulating the rate 

 of flow of the water, to arrange that the temperature of the surface from which this 

 heat was lost was nearly identical with that of the air, and the loss was thus 

 reduced to so small an amount that it could be neglected. By means of tbermo- 

 junctions of copper and platinoid wires, soldered into holes about 3 millims. deep in 

 the edges the discs, U and M, and in holes 2 centims, deep in the edge of the disc, L, 

 the temperature of the upper disc, and the differences of temperature between the 

 upper and middle, and between the upper and lower discs, were found by balancing 

 the thermo-electric E.M.F. in each case against the requisite fractional part of the 

 E.M.F. of a Leclanche cell, which had been compared with a standard Clark cell. It 

 was found that for small temperature differences the E.M.F.s so determined, could be 

 taken as proportional to the differences of temperature. 



Fig. 12. 



v c 



U 



From the difference of temperature on the two sides of the rubber sheet, and the 

 thickness area and conductivity of the rubber, the amount of heat entering the 

 liquid could be calculated, and if the thickness of the liquid, the area of flow, and the 

 difference of temperature are known, the thermal conductivity of the liquid can be 

 found. 



The curvature of the lines of flow in the liquid near the edge of the disc, M, will 

 render the area of flow in the liquid greater than in the rubber, and a small correction 

 would have to be applied if the thermal conductivity were made to depend on that 

 of the rubber. It is better, however, to base the determination on the conductivity 

 of water, which can be substituted for the liquid, and tested under the same con- 

 ditions. In this case the areas of flow in the two liquids may be assumed to be equal, 

 and the calculation simplified. 



8 H 3 



