l8 ELECTROMOTIVE FORCE OF IRON AND OCCLUDED HYDROGEN. 



Two other calculations will be given to illustrate more fully the changes 

 to be expected. The data for these were found in a book u which is still 

 quoted as an authority by engineers, although published as long ago as 1879. 



In the first of these (p. 24) the test-pieces were taken " from a bar of 

 remarkably pure, refined, and uniform iron." The average of nine tests, 

 very uniform in their results, gave: Breaking stress per square inch, 55,489 

 pounds ; elongation, 23.9 per cent. The diameters of the test-pieces were 

 0.974 inch. Calculating as before, we get 0.0034 volt as the change to be 

 expected. 



In the second one the iron is described as " very soft and ductile." The 

 average of four tests gave: Stress per square inch, 45,873 pounds; elonga- 

 tion, 29.7 per cent. The diameters were 1.000 inch. These data give 0.0035 

 volt as the expected rise. 



The results of all these calculations should be divided by 2, since Barus 

 showed that only 50 per cent of the work done becomes potential. These 

 calculated values, never exceeding 1.8 millivolts, represent the maximum 

 values for the increase of potential values which may not be actually 

 attained, because it is by no means certain that all the energy thus stored is 

 available as free energy. Moreover, the distribution of the strain between 

 surface and interior is uncertain. This is indicated by the following state- 

 ment of Burr : 20 



It has been found by experiment that bars of wrought iron which are apparently 

 precisely alike in every respect, except in area of normal section, do not give the 

 same ultimate tensile resistance per square inch. Other things being the same, bars 

 of the smallest cross section give the greatest intensity of ultimate tensile resistance. 



This is due to the fact that shearing strains increase with greater cross 



section. And also, 21 



If the tensile stress is uniformly distributed over each end of the test-piece, it will 

 not be so distributed over any other normal section. Since lateral contraction takes 

 place, the exterior molecules of the piece must move toward the center; but if this 

 motion exists, the molecules in the vicinity of the center must be drawn farther apart, 

 or suffer greater strains, than those near the surface. Hence the stress will no longer 

 be uniformly distributed, but the greatest intensity will exist at the center and the least 

 at the surface of the piece. These effects will evidently increase, with a given form 

 of cross section, with the area. 



Smith, 22 in pointing out the strengthening effect of drawing wire has said : 



" In the case of the wire drawn through three holes the tenacity of the 



18 " Experiments on the strength of wrought iron." Report of the Committee of 

 the United States board appointed to test iron, steel, and other metals. Commander 



L. A. Beardslee, U. S. N. Abridged by William Kent. New York, 1879. 



2,1 Elasticity and resistance of the materials of engineering, p. 218. New York, 1903. 



21 Ibid., p. 206. 



22 " Wire, its manufacture and uses," p. 58. 



