>70 PROCEEDINGS OP THE AMERICAN ACADEMY. 



JSvaluation of the Thomson Effect. 



I. Consider the sections from lead no. 4 to leud no. 12. 4 and 12 

 may replace x^ and x.i (see page 360). 

 Then : — 



(1) R^ — R - .000005813 (from column VII., page 369). Y = 35 ; 



4 to 12 4 to 12 



hence Y2 IR^-R \ = 35- x .000005813 = .00713 watts. 



V 4 to 12 4 to 12/ 



(2) m — H = .00965 (from column XL, page 369). 



4 to 12 4 to 12 



(3) The change in the temperature gradient at no. 4 on reversal of 

 the heating current, determined from curve c d, Fig. 3, =:0°.752 per cm. 



Area of cross section at no. 4 = .00713 sq. cm. 



The thermal conductivity at no. 4 = .89, on the assumption that 

 thermal conductivity rn .87 (1 + .0004 t). 



Hence k 



= .752 X .00713 X -89 X 4.2 = .0200 

 watts 



1_ \d x) \d X, 



Similarly /?;io \ (—] - (— V = -.400 X -00724 X -95 X 4.2 = 

 \_\dx) \dxj _\ _. one watts. 



The temperature at no. 4 = 41°. 

 The temperature at no. 12 = 232°. 



Hence the ditference of temperature between the ends of the section 

 is 191°. Substituting in the equation of page 360, we have 



2 o- (232-41) X 3.5 = .00965 - .00713 + .0200 - (-.0116) 



.00965 — .00713 + .0200 + .0116 nnnno^^Q .. 

 .-. o- = ■ ■ = .00002oo3 watts. 



2 X 3.5 X 191 



= .00000608 calories per second. 



Mean temperature of section =116° C. 



Expressing the result exactly: — When an electric current of 10 

 amperes flows, in the same direction as the flow of heat, for 1 second 

 through a section of a copper bar whose ends differ by 1° C. and whose 

 mean temperatuie is 116° C, the Thomson Effect will be represented by 

 the liberation of .00000608 calories. 



The work of calculating the value of the Thomson Effect for other 

 parts of the bar at different temperatures is precisely similar to the 

 above ; so a summary only is given in the following table : — 



