J. O. Thompson — Law of Elastic Lengthening . 33 



ments of the thermal effects produced by the stretching of 

 wires find no inaccuracy in the law. Neither did Morin,* 

 although the distance between the upper and lower marks on 

 his wires amounted to 21 m. 



Millerf too endeavors to prove that the elastic lengthening 

 is proportional to the force. As far as I am aware, the old 

 law has generally been regarded as axiomatic, and no one has 

 ever made the attempt to replace it by a new one. 



At the time of some experiments here made by Mr. Strad- 

 ling this subject came up for discussion, and Prof. F. Kohl- 

 rausch invited me to investigate it in a series of careful experi- 

 ments. In this place I desire to express my sincerest thanks 

 to my honored teacher, Prof. Kohlrausch, for his advice and 

 help in the course of these investigations. 



In the first place it should be noticed that there is no good 

 a priori reason why Hooke's law should be strictly true. 

 When one examines the case closely it seems highly probable 

 that deviations from the law should appear. When a wire is 

 subjected to a tension the relative positions and distances of 

 its molecules are altered, its thermal state is changed, and it 

 becomes in these respects a new body. Accordingly that there 

 should be in one and the same body a definite relation between 

 the modulus of elasticity and the amount of deformation ought 

 to occasion no surprise. 



Since the experiments of others have amply proved that 

 deviations from the old law of lengthening, if any really exist, 

 must be very small, it is evident at the outset that the measure- 

 ments, in order to detect these deviations, must possess a high 

 degree of accuracy. The following experimental conditions 

 must accordingly be secured : The wire must be long and free 

 from all curvature. The temperature of the place where the 

 experiments are made must remain approximately constant 

 during an experiment.- Further, account must be taken of 

 several secondary phenomena the most important of which are 

 the thermal effects due to the changing volume of the wire, 

 and the elastic after-effect. The influence of these secondary 

 phenomena will later be separately discussed. 



The wires made especially for these experiments by Herr 

 Obermaier of Nurnberg, gave me the first of the above men- 

 tioned conditions. The tower of the Physical Institute of the 

 University of Strassburg, permitted the use of wires of the 

 requisite length; and, since it was situated on the north side 

 of the building, it possessed such a uniform temperature that 



* Morin, Comptes Rendus, liv, (1862), p. 235. 



f Miller, Aus d. Sitzungsber. d. math-phys. Classe der k. bayer. Akd. d. Wiss, 

 1882, Heft 4. 



Am. Jour. Sol— Third Series, Vol. XLIII, No. 253.— January, 1892. 

 3 



