344 Dr. C. Barus on the Relation between the Thermoelectric 



fact that the wire is kept in a state of continual tension by the 

 spring d K, and from tho particular method of chilling, the 

 wires remain straight after being hardened. (3) The very 

 slight oxidation noticeable on the hard wires is probably due 

 only to the contact of hot steel and water in the act of harden- 

 ing. Disadvantages, however, arise from the fact that the use 

 of tho apparatus is confined to thin bars, and that the wires 

 obtained may be in a condition of circular magnetization. 

 This would partly prevent their employment in subsequent 

 magnetic experiments. The difficulty may, however, be 

 avoided by breaking the galvanic circuit a little before open- 

 ing the faucet. 



III. Methods of Measuring the Hardness of Steel Electrically. 



(a) Thermoelectric Position and Hardness of Steel. 



In this place it will be expedient to leave the special consi- 

 deration of steel for a moment, turning our attention to the 

 electromotive force of a thermoelement composed of any two 

 different metals A' and A ff . 



Kohlrausch has shown* that the phenomena included under 

 the head of thermoelectricity can be explained on the hypo- 

 thesis that the heat-current is always accompanied by an 

 electric current whose intensity is proportional to the number 

 of caloric units passing the same section. He thus arrives at 

 an expression for the electromotive force between any two 

 metals (A' and A"), which, if for simplicity we suppose the 

 cold end to be kept at zero f, has the following form : 



E r =(y-y')r(i+/«), 



where E T is the electromotive force corresponding to a differ- 

 ence of temperature t of the ends, ($' — § r/ ) a constant specific 

 for the combination. 



This expression of Kohlrausch is very convenient, inasmuch 

 as it allows us to separate the actual electromotive force into 

 two terms, of which the first, §'t(1 +/(t)), is dependent only 

 on the metal A and t, the second, ^(l +fr), only upon A" 

 and r. 



Now we know that the thermoelectric position of a metal is 



case the wires should be cooled without breaking the current, in the 

 second the tube surrounded by a coil through which, during the act of 

 hardening, a powerful galvanic current Hows. See Holtz, Wied. Ann, vii. 

 p. 71, 1879. 



* Pogg. Ann. clvi. p. 601, 1875. 



t The thermoelectromotive force being, according to Tait, Avenarius, 

 Hankel, and others, a function of the temperatures of the two ends. 



