20 ELECTROMOTIVE FORCE OF IRON AND OCCLUDED HYDROGEN. 



The weights were then added to 48 kg., the last ten still causing a slight 

 swing to the right. A 13 kg. weight was then added to the load, and the 

 wire lengthened rapidly and would have undoubtedly broken had not the 

 support stopped the basket. The galvanometer swung 8.5 divisions to the 

 left, indicating a rise of about 0.004 volt, and quickly swung back again. 



These temporary changes and the change in sign which they undergo 

 are highly interesting. The most obvious explanation of the first effect of 

 diminished potential is to ascribe this to heating incidental to pulling, and 

 experiment gave the idea some support. Heating the wire to redness with 

 a small gas flame just above the surface of the liquid gave a somewhat 

 variable electromotive force, the mean value being about 0.015 volt below 

 the initial one an effect about like the first effect noted above. When 

 cold the wire returned to the initial value. This is advanced as a suggestion 

 rather than a definitive explanation, however. 



The decreasing temperature effect due to the successive additional stresses 

 may have been due to the larger percentage of storing as potential energy as 

 the elastic limit is approached and the final excess of potential may have 

 been due to the sudden manifestation of this increased potential at the point 

 of fracture. 



Nevertheless, these conclusions are highly hypothetical. The results do 

 not permit of conclusions broader or more definite than these. On the other 

 hand, they afford the opportunity of correcting two pieces of work which 

 have been widely published and whose incorrect results have been given 

 general credence. The subject is of such great importance to modern 

 engineering that, naturally enough, engineers were the ones attracted to its 

 investigation. M. P. Wood, in his book " Rustless Coatings ; Corrosion and 

 Electrolysis of Iron and Steel," M under the heading " Corrosion Increased 

 by Stress " (p. 348), quotes three men as entitled to a hearing on this subject. 



The first of these, Thomas Andrews, 26 carried out experiments far from 

 satisfactory from the electrochemical point of view, because he used a solu- 

 tion of common salt as an electrolyte, and prepared the two pieces of iron 

 for comparison in different ways. The observed difference in potential is 

 quite as probably to be ascribed to the oxidation effect already discussed as 

 to the effect of strain, although a part of the effect may have been due to 

 strain. 



The second paper quoted by Wood is " An Experimental Study of the 

 Corrosion of Iron under Different Conditions," by Carl Hambuechen. 27 Of 



25 Wiley & Sons, N. Y., 1904. 

 M Proc. Inst. Civil. Engin. 118, 356-374 (1894). 



"Trans. Amer. Soc. Mechan. Engin., 22, 816-821 (1901) ; or Bull. Univ. Wisconsin, 

 Engin. Series, vol. 2, No. 8, (1900). 



