230 PROCEEDINGS OF THE AMERICAN ACADEMY. 



construction are often based upon hysteresis diagrams or magnetization 

 curves, which are supposed to show the magnetic characteristics of 

 different specimens of iron and steel under given excitations, and, if the 

 forms of these figures depend very much upon the manner of applica- 

 tion of the excitations, it is essential that the observations be made 

 under conditions approximately the same as those under which the iron 

 will have to do its work. It is important, therefore, that the engineer 

 should know how closely the test conditions and the working condi- 

 tions must correspond, and a number of investigators have studied the 

 subject. 



At the outset the subject may be divided into two parts : first, the 

 magnetization of large solid masses of iron ; second, the magnetization 

 of finely laminated iron. 



For the former it is almost unanimously agreed that, if the change 

 in the magnetizing field is always the same in direction, in going from 

 one value of the magnetic field to another, the manner in which 

 that change is produced affects the total change of flux in the iron. 

 Fromme,2 Lehman,^ Gumlich and Schmidt,* Heyse,^ Rilcker ® and 

 B. 0. Peirce "^ have obtained results with various shapes of iron, such 

 as toroids, ellipsoids and rods, which show this difference in the flux 

 change to a greater or less extent. Rilcker, for instance, found that 

 in the case of soft iron it might amount to 30 per cent of the total 

 flux change. For hard material the percentage was smaller. 



In some early experiments I used a large, massive cast iron magnet 

 weighing about 1500 kilogrammes. Its general shape is shown in Fig- 

 ure 1. The base is of rectangular cross-section, 40 cm. by 20 cm., 101 

 cm. long and 80 cm. high ; the arms are cylinders of steel 25 cm. in 

 diameter and are capped by rectangular pole pieces 4.5 cm. thick and 

 580 cm. in area. Four coils having a total of 2823 turns and a 

 resistance of 12.4 ohms served for the magnetization. 



The time required for the current to reach a constant value in such 

 a system is very appreciable. Two to three minutes was not at all 

 unusual. Such long, slow flux changes taking place in the iron could 

 not be measured accurately by the long-period galvanometer ^ that had 

 been constructed for this sort of work. However, it was found that 



2 Wied. Ann., 43, 181 (1891); 44, 138 (1891). 



3 Ibid., 48, 422 (1893). 



* Elektroteknische Zeitschrift, 21, 1900. 



5 Inaug. Diss. Halle, p. 33, 1901. 



6 Inaug. Diss. Halle, 1905. 



' These Proceedings, 43, 155 (1907). 

 8 These Proceedings, 44, 1908. ' 



