598 PKOCEEDINGS OF THE AMERICAN ACADEMY. 



Thomson effect coefficient, v : 



r — 757 X 10-^° mean, from IS*" to 90°, 



approximately \ - 715 " " " " " 51°, 



[ - 793 " " " 51° " 90°, 



[a mean rise of — 2 X 10~^° per degree]. 



We have now to add for the same specimen of iron : 

 Temperature coefficient of thermal conductivity between 115° and 

 204° C, referred to the value of the conductivity at 115° = — 0.00068, 

 approximately. Electric resistance, absolute : 17260 at 100° and 

 26140 at 218.2°, with a mean temperature coefficient 0.00661 (on the 

 basis of the 0° value of the resistance) between 100° and 218°. 



Thomson effect coefficient, v (— a- -^r T, where o- is the fraction of a 

 calorie absorbed per second in the Thomson effect by a current of 10 

 amperes flowing from a cross-section at temperature (T— ^) absolute 

 to a cross-section at temperature (T + h) absolute), from a combina- 

 tion of our new observations with our old ones, approximately (see 

 pp. 614, 615). 



- (717 + 2 (T- 305)) X 10-^ 



from 32° C. to 182° C. (The rate of change of v here indicated would 

 make the tangent of the angle of inclination of the iron line on the 

 ordinary thermo-electric diagram about 40 per cent greater at 182° C. 

 than at 32° C.) 



Battelli, who worked about twenty years ago with a specimen of iron 

 which he does not describe, except as to its shape and size, got values 

 from which we can deduce the approximate law 



,-10 



V = - (283 + 0.34 (T- 326)) X 10 



from 53° C. to 308° C. 



Lecher, who also in his recent paper fails to describe the quality 

 of the iron with which he worked, gets o- as — 278 X 10~', that is, 

 V — — 855 X 10~^°, at 52° C, and gives a formula which, when allow- 

 ance is made according to the data of B^de, for the change of specific 

 heat of iron with change of temperature, indicates a considerable 

 increase, about 12 per cent, in the numerical value of r between 50° and 

 130°, a nearly constant value from 130° to 160°, a slight numerical fall 

 between 160° and 180°, and an increasing rate of fall with further rise 

 of temperature. 



It appears, then, that there is general agreement among those who 

 have worked directly on the Thomson effect in iron that the line for 

 this metal on the thermo-electric diagram should be one of increasing 



