256 Mr. E. Taylor Jones on 



with the magnetizing current. With small currents the lifting- 

 power increased more rapidly than the magnetometer- deflex- 

 ion, but with stronger currents more slowly, the corresponding 

 curve ultimately approaching a horizontal asymptote. Also 

 the lifting-power was proportional neither to the current nor 

 to its square, but was represented approximately by a function 

 of the form b tan -1 (ac), where c is the current, and a, b 

 constants. 



The magnetizing forces, however, used in these experi- 

 ments could have been but small, since the straight magnet 

 was never more than half saturated, and its magnetization 

 was always nearly proportional to the current. 



The next experiments were made in 1881 by Werner 

 v. Siemens*. The electromagnets were made by cutting an 

 iron tube in a plane through the axis, and winding both 

 halves with wire. The interfaces were carefully ground 

 together. A ballistic galvanometer and secondary coil were 

 used to measure the induction. From the total induction- 

 " throw " was subtracted the throw caused by breaking the 

 current in the coil after the iron was removed. The resulting 

 differences are therefore proportional to the intensity of 

 magnetization, not to the induction. 



The result showed that the lifting-power was approximately 

 proportional to the magnetic moment per unit volume, but 

 that the ratio somewhat increased as the current was increased. 

 Similar results were obtained with electromagnets formed by 

 cutting a circular iron tube in the plane through the greatest 

 section. 



Siemens believed the above law to be established, and 

 attributed the deviations to residual magnetism, and to im- 

 perfect contact of magnet and keeper due to bending, the 

 imperfect state of the surfaces, and other mechanical causes. 

 These were the probable causes of much greater errors than 

 those believed to exist. 



In 1882 Wassmuthf experimented with magnets similar 

 to those used by v. Waltenhofen. The induction was measured 

 ballistically, and the ends of the magnets were ground plane 

 and polished. The magnetic moment per unit volume was 

 calculated from the induction-current and the results com- 

 pared with a theory given by Stefan J, according to which 

 the stress is proportional to the square of the magnetic 

 moment per unit volume at the surface of contact. Wassmuth 



* Wied. Ann. xiv. p. 640 (1881). 

 t Wien. Ber. lxxxv. p. 327 (1882). 

 X Ibid, lxxxi. ii. p. 89 (1880). 



