THE WORK OF BARUS. IJ 



itself to its original dimensions. Beyond the elastic limit, the custom in 

 wire manufacture, of returning the wire perhaps half a dozen times to the 

 annealing pots to be softened, speaks for itself. By heating to 300 to 400 

 for several hours " the molecular tension in the wire caused by drawing " 

 is released. 17 The molecules, given the opportunity through increased mo- 

 bility at higher temperatures, may be supposed to return to their normal 

 relations to each other and plasticity is restored to the iron. 



Further evidence in this direction is furnished by a piece of work done by 

 Barus, entitled '' The Energy Potentialized in Permanent Changes of Molec- 

 ular Configuration." 18 He pulled wires of various metals to the point of 

 failure, determined their rise in temperature by means of a thermopile, and 

 subtracting the heat evolved from the total work done on the wires, obtained 

 values for the amounts of energy which had become potential in the metals. 

 He says : 



To summarize, it appears that as much as one-half of the work done in stretching 

 up to the limit of rupture may be stored up permanently; that the amount of work 

 thermally dissipated varies considerably with the metal acted upon, being very large, 

 for instance, in copper (75 per cent) and smaller in the case of iron (50 per cent) ; 

 that in the case of the same given metal the work is largely potentialized during in- 

 cipient stages of strain, and very largely dissipated during final stages of strain. When 

 stress of a given kind is applied to different metals, the total amount of energy which 

 can be stored per unit section, per unit of length up to the limits of rupture, may 

 therefore be looked upon as a molecular constant of the metal. 



In a table he gives results showing that in an iron wire of 0.136 cm. 

 diameter, stretched almost to the limit of rupture, at least 2 megergs per 

 centimeter are potentialized, about the same amount having been dissipated as 

 heat. Knowing this value, it is possible to calculate the maximum rise in elec- 

 tromotive force theoretically required by such an increase in the free energy 

 of the metal, supposing that all this work were available as free energy. The 

 general formula for such a calculation is : 



WE 



Atz = 



9658OW 

 in which 



W = the work in joules done per centimeter of the wire ; 



E = the electrochemical equivalent of the metal, in grams ; 



m = the weight of the metal per centimeter of wire ; and 



W X E/m the work done per gram equivalent of the metal. 



Substituting values found by Barus, we have : 



4* = 0.432 X 27.95/0.1 1 14 X 1/96580 = 0.0011 volt. 



(The radius of the wire was 0.068 cm. and its density 7.68.) 



17 " Wire, its Manufacture and Uses," J. B. Smith, pp. 30, 54, and 56. 

 18 Amer. Journ. Sci. (3), 38, 193 (1889); also U. S. Geological Survey Bulletin No. 

 94, p. 101 (1892). 



