

AND LIQUIDS AND THEIR VARIATION WITH TEMPERATURE. 413 



E 2 = (ft - ft) e' (\ + /' ftT~ft) 



where e and/' are constants. 



If at the junctions at temperature ft, the two circuits are joined in series, the 

 electromotive force in the circuit is the difference of the above, and if we write it 

 E;j 10 we have 



E 210 = (ft - 6.} e' (1 +/' 6, + ft) - (6, - ft) e (1 + /ft + ft). 



Let ft become identical with ft, and let E 1IO now be the electromotive force in circuit. 

 Then 



E 110 = (ft - ft) e (1 +/'.ft + ft) - (ft - ft) e (I + /ft + ft). 

 Therefore 



E 210 - E 110 = (ft - ft) e' (1 +/' ft + ft), 

 or 



ft - ft = (E 210 - Euo)/e' (1 +/' ft + ft), 



which furnishes a means of determining ft ft from observations of the electromotive 

 force when the circuits are in series, and of the electromotive force iu the same 

 circuits when the temperatures ft and ft are identical. In the tables which follow 

 the quantity E 210 is indicated by the term " Observed," and E 210 E uo by the term 

 "Reduced." - 



In the experiments, ft is the temperature of the system of mercury cups (fig. 7, 

 p. 401), indicated in what follows by J. The junctions in the discs are indicated by 

 the letters U, M, C, B, for upper, middle, cover, and junction inside the air bath 

 respectively. 



Method of Experimenting. 



In carrying out the test of a disc of any substance, the copper discs, between 

 which the disc to be tested was to be placed, were first brought together, and the 

 total thickness of the combination measured by means of a micrometer wire gauge. 

 The thickness of the disc to be tested was then measured, the disc placed in the 

 proper position between the copper discs, glycerine being used to improve the 

 contacts, and the total thickness measured. The differences of the observed thick- 

 nesses is the thickness of the disc of the material used, plus that of the two layers 

 of glycerine, and we know, therefore, the thickness of the glycerine layers, and can 

 calculate their effect on the flow of heat. 



The combination was then suspended in the centre of the air bath, with the plane 

 surfaces of the discs vertical. An electric current was then sent through the heating 

 coil of the discs, which increased in temperature and began to lose heat to the bath. 



