104 



HEAT. 



in accordance with Fourier's calculation founded upon the law of con- 

 ductivity, and thus proved that the law was the same as for solids. 



Other experimenters have since made determinations of the con- 

 ductivity of liquids. Thus Lundquist, employing Angstrom's method, 

 obtained for the conductivity of water at 40'8 K = -00156. H. F. Weber * 

 used a disc method, the liquid layer, '231 cm. thick, lying between two 

 copper discs 16 cm. diameter, and being kept in position by capillarity, 

 or by a glass rim to the lower plate. The upper plate was about 1 cm. 

 thick, and was assumed to be at one temperature throughout, the 

 temperature being given by a thermo-electric junction attached to it. 

 The lower plate was half the thickness of the upper. When the whole 

 apparatus was at one temperature it was suddenly placed on a block of 

 ice at 0, in an enclosure at 0, and the rate of cooling of the upper 

 plate was observed. The mathematical deduction of the conductivity 



Copper 



Heating 



Copper C, 



Glass G 



Copper C a 



Ebonite] Liquid X \Ebonite 



Copper C 3 



FlG. 71. Lees's Disc Experiments on Liquids. CiC 3 C 2 , copper discs ; 

 G, glass disc ; X, liquid surrounded by ebonite ring. 



from the rate of cooling is not simple, and Weber's calculation has been 

 criticised and amended by Lorberg, whose recalculation gives a value for 

 water nearly 10 per cent, greater than that of Weber. But, where so 

 much uncertainty exists, it is sufficient to state Weber's results for 

 water in the form 



* t = -0012(1 + -0080 



where the assumption is made that the conductivity changes uniformly 

 with the temperature. 



Other experimenters have obtained values of the same order of 

 magnitude. We shall describe only the method used by Lees,f a 

 modification of his disc method already described. 



The liquid tested filled the cavity made by an ebonite ring between 

 two copper discs. The principle of the method consisted in sending a 

 known quantity of heat down through the upper of these discs, and from 

 the temperature of the lower disc determining the conductivity of the 

 liquid layer. The heat thus sent down was determined by putting a 



* Chree, Phil. Mag., xxiv., 1887, p. 1, gives an account of this and other work. 

 t Phil. Trans., A., 1898, p. 399. 



