Electromagnetic Stress, 255 



has succeeded in determining accurately what function of 

 the intensity of magnetization or of the induction the stress 

 really is. In order to show how the methods of experiment- 

 ing have developed, and how they have led to those of the 

 present investigation, I will describe shortly the chief experi- 

 ments that have so far been made*. 



As long ago as 1833 Fechnerf made a series of measure- 

 ments of the weight necessary to separate a horseshoe magnet 

 and its keeper, measuring the current by swinging a mag- 

 netic needle within a coil through which the current passed. 

 He found a rough proportionality between the limiting weight 

 and the current. 



Somewhat more accurate experiments made in 1839 by 

 Lenz and Jacobi { showed that this proportionality did not 

 exist. In the same year Joule §, measuring the magnetizing 

 current with an ingenious " current-weigher," found that for 

 small currents the lifting-power of an iron electromagnet 

 was proportional to the square of the magnetizing current, 

 but that for stronger currents the lifting-power increased 

 more slowly and ultimately reached a maximum value of 

 140 lb. per square inch ; twelve years later ||, however, using 

 a more powerful electromagnet, he found the maximum value 

 to be 175 lb. per square inch. 



In 1852 DubH found that the lifting-power was propor- 

 tional neither to the current nor to the square of the current, 

 but to some intermediate function, and for strong currents 

 reached a maximum. 



Much more accurate experiments were made in 1870 by 

 v. Waltenhofen**. Two similar bars of iron with plane ends 

 were bent approximately to semicircles and wire wound evenly 

 on both. One bar was fixed rigidly in a frame and the other 

 one pulled up from it, the necessary force being measured by 

 a spring-balance. A third bar and coil, similar to the other 

 two but straight, was set up with a compensating-coil in the 

 East and West line through a magnetometer-needle. All 

 these coils were in circuit with a galvanometer for indicating 

 the current. 



The stress between the bent electromagnets could thus be 

 compared with the magnetic moment of the straight one, and 



* Cf. also Wiedemann, Eleh. iii. Bd. 3, pp. 632-654; du Bois, Mag- 

 netische Kreise, sect. 105-110 (1894). 

 t Schweigg. Journ. lxix. (1833). 

 X Pogg. Ann. xlvii. p. 415 (1839). 



§ Phil. Mag. Dec. 1851. || Ibid. Jan. 1852, 



^1 Pogg. Ann. lxxxvi. p. 553 (1852). 

 ** Wien. Ber. lxi. p. 739 (1870). 



