J. 0. Thompson — Law of Elastic Lengthening. 47 



were gradually increased to a maximum of 18 kg. and with- 

 out exception the results obtained were similar to those which 

 I have reported. The reasons however why I preferred to use 

 fine wires are first, because in these the thermal effects vanish 

 more rapidly, and second, because the loading and unloading 

 can be done in shorter time, and thus the after-effect is more 

 completely eliminated. 



Earlier Investigators. 



Since these investigations have yielded results which are at 

 variance with those hitherto obtained, it is desirable to search 

 for an explanation of the discrepancy. It is evident that the 



rp 



presence of curves and bends in the wire would give to — too 



large a value which would gradually decrease as the wire 

 grew straight. This circumstance would account for Strad- 

 ling's remark* according to which the modulus of elasticity 

 would increase with the tension, and is the probable explana- 

 tion of the fact that Wertheimf in the case of annealed plati- 

 num finds the modulus increasing with the weight. Further 

 the almost universal result obtained by other investigators, 



namely that — has a constant value, can be explained in the 



very same way. The diminution of the modulus alone con- 



rp 



sidered would make — larger, while the gradual straightening 



Jr ' 



of the wire would tend to make it smaller. The combination 

 of these two factors could easily lead to the conclusion that 



x 



ry* 



within the limits of errors of observation — has a constant 



value. 



Discussion of Results. 



"Whatever the nature of elastic force may be, whether it is. 

 an essential property of matter, or a derived property which 

 can be accounted for by the rotation of atoms, the fact was 

 established by Wertheim that between the elasticity of a body 

 and its density there exists an intimate relation. Conclusion I 

 of his investigation is as follows 4 " The coefficient of elas- 

 ticity is not a constant for one and the same metal. All cir- 

 cumstances which increase the density make it larger, and con- 

 versely." These words justify a deduction which .Wertheim 



*Stradling, Wied. Ann., xli, p. 332, 1890. 



f Wertheim, Pogg. Ann. Erganz., ii, p. 46, 1848. 



% Wertheim, 1. c. p. 69. 



