460 Mr, F. W. Jordan on the Direct 



As an instance of the calculation of P : — 

 At 22°*4 C, 



0*125 2 x 2*78 4- (>130 2 x 2-69 2 

 " 1'466 4- 1-431 + 1-366 + 1-332 * 

 P = 0*01590 volt from bismuth to copper. 



In .this case the excess of temperature of cylinder above 

 enclosure = 3° nearly. 



The resistance of the bismuth rod between copper cylin- 

 ders = 1730xl0- 6 ohm. 



Length of rod — 1'76 cm. 

 Mean cross-sectional area of rod = 0*1.54 sq. cm. 



Specific resistance at 19°*5 = 151 X 10 -6 ohm. 



The effective resistance of the copper lead to cylinder was 

 deduced from the change of the compensating current through 

 the junction on reversing the current through the heating 

 eoil. 



Let Cj and C 2 be the values of the junction currents ; 

 c the current through the heating coil; 

 r t the effective resistance of the copper lead which is 

 traversed by both the junction and heating 

 currents; 

 ?' 4 the effective resistance of the bismuth rod at this 



junction: 

 7\; the total effective resistance of the copper lead and 

 bismuth rod at the other junction. 

 Then the following equations hold for currents C] and c in 

 the same direction through the copper lead : — 



Q + (C 1 + c)V i +C 1 V 4 -PC 1 =V 1 ... (7) 



dS + Pd^i. . . . (8) 



and for a reversed current c : — 



Q + (C 2 -o) 2 r 3 + C 2 V 4 -PC 2 = ^^ ... (9) 



C 2 V 2 + PC 2 =V 2 . . . (10) 

 These equations give 



r 3 (C-C 2 +2c)(C 1 + C 2 ) = ^±^^ 2 (C 1 2 -C/) + P(C 1 -C 2 )} 



-04 + »' 2 )(CY 2 -C 2 2 ). 



To obtain an approximate value of r 3 , -— — 2 may be 



^2 



written =2: and since the results iu the table show that the 



