22 ELECTROMOTIVE FORCE OF IRON AND OCCLUDED HYDROGEN. 



volt. This rise would have been ascribed by Hambuechen to strain. Further 

 experiment could add nothing to the argument. It will be noticed that even 

 finally the potential was very far from the value in ferrous sulphate, 0.765. 



Hambuechen's results are quite as irregular, as one would infer from these 

 considerations. For example, the change in electromotive force, caused 

 supposedly by tensile stress, varies in two cases from 0.004 to 0.056 volt, and 

 yet in the former case the load per square inch was actually 600 pounds 

 greater than in the latter. Hambuechen realized that some of his observed 

 potentials were much larger than were to be expected, and therefore ex- 

 plained that " One of the assumptions it was necessary to make is that the 

 stress equally affected the entire cross section, which is by no means the 

 case, for it is the outside layer which is affected the most and would there- 

 fore give higher values of electromotive force than the calculated amounts." 

 This is exactly the opposite assumption to that demanded by the reasonable 

 explanation of Burr already quoted (p. 18). 



From the theoretical point of view, the matter appears, then, to be more 

 complicated than might be inferred from a hasty survey of it. One can not 

 safely conclude that the potential on the surface of a wire is an index of the 

 free-energy change in the interior of the wire ; and one can not assume that 

 the electrochemical behavior of wire while under stress is the same as when 

 the stress is removed. Still further, one can not but believe that the perma- 

 nent strain caused by a stress (that is to say, the energy potentialized by a 

 stress) will differ in the case of impure iron from that in the case of pure 

 iron. Moreover, it is by no means certain that energy stored in iron by 

 change of internal structure is available as free energy. 



Taking all these considerations together, and weighing also the electro- 

 chemical defects in some of the papers just discussed, it is apparent that 

 none of these investigations furnish conclusive evidence concerning the 

 magnitude of the changes of potential to be noticed on stretching a rod or 

 wire. It is apparent, moreover, that this change is smaller than is usually 

 supposed, although it is probable that a slight change of electromotive force 

 is caused by a tensile strain of impure iron. 



Qualitative evidence on this point is furnished by a paper entitled " The 

 Effect of Strain on the Rate of Solution of Steel," published by Barus in the 

 Bulletin 94 of the U. S. Geological Survey (p. 61), already quoted. He 

 experimented with soft steel, hard-drawn steel, and the latter annealed ; he 

 determined the rate of solution by loss of weight, and he concluded his 

 report as follows : " Summarizing the above results as a whole, it follows 

 that the rate of solution of drawn steel is greater than the rate of the same 

 homogeneous metal similarly circumstanced." In applying the results it 



