412 



DR. C. H. LEES ON THE THERMAL CONDUCTIVITIES OF SOLIDS 



rature is known approximately, the value of the divisor may be found from the upper 

 of the preceding curves, and the temperature found more accurately. If necessary 

 this new value of the temperature may be used to 6nd the divisor more accurately, 

 and from it a second approximation to the temperature. 



To obviate the necessity of determining the temperature of the disc in this way by 

 two stages, a table of divisors was constructed having a double entry, one, the 

 temperature of the junction at the mercury cups, the other the resistance of the 

 potentiometer for a balance. The form of the table will be seen from the part of it 

 given below : 



To determine the temperature of the upper junction when the junction in the 

 mercury cups had a temperature of say 18 C., and the resistance for a balance was 

 say 3 ohms, we find from the table that the divisor is '388, and the temperature 

 difference between the junctions is 3/'388 = 773, and the temperature required 

 therefore = 2573 C. 



The same plan was adopted for the junction in the air bath. 



Tlieory of Observations with Thermo-junctions in Series. 



If two homogeneous wires of materials a and b have their ends soldered together, 

 and the two junctions are kept at temperatures #1 and 6 , the electromotive force Ej 

 in the circuit is given by an equation of the form 



where e and /are constants. 



If the junctions of a second pair of materials, a and b, where a' differs slightly 

 from a in thermo-electric properties, are kept at temperatures 6 t and 6 , we have 



