284 PROCEEDINGS OF THE AMERICAN ACADEMY. 



however, that the thickness of the photographed line obscures some- 

 what the slow changes when the exciting current has nearly reached 

 its new value, and in the very sensitive instruments sometimes required 

 for use in a secondary circuit there is a small but occasionally trouble- 

 some lag just at the beginning of the motion. For all ordinary pur- 

 poses this method is wholly satisfactory if not always easy or convenient 

 to carry out. 



Such fluxmeters as I have been able to procure, though admirable 

 in many ways, have not been so free from crawling, due apparently to 

 the paramagnetic properties of their copper coils, that their indications 

 can be trusted for very slow magnetic changes. If the fluxmeter coil 3 

 is not wound on a metal frame, the mutual damping caused by the ac- 

 tion of currents in the coil, and the core which it surrounds, are not 

 always effective unless the resistance of the exterior circuit is small, 

 and this frequently makes an instrument which works very well for one 

 piece of work, nearly useless for another. 



When the excitation of the core of a large electromagnet initially in 

 a given magnetic condition and under a given excitation is changed 

 by a predetermined amount, it sometimes happens — as is well known 

 — that the resulting change in the magnetic flux through the iron de- 

 pends somewhat upon the manner in which the exciting current is 

 changed ; that is, the flux change is different when the current in the 

 magnet coil is changed very gradually or in short steps from what it is 

 when the change is made very suddenly. This difference is generally 

 small, and seems to depend upon a variety of circumstances 4 in a way 

 not yet very well understood, but it must be determined for every large 

 magnet if the behavior of the core under given conditions is to be 

 predicted with any great accuracy. 



I have recently had occasion to inquire how the changes of magnetic 

 flux in each of a number of large cores, of which two are represented 

 by Figures A and B, corresponding to given changes in the current in 

 the exciting coil, depend upon the manner of growth of that current, 

 and since such oscillograph records as I was able to make were not 

 wholly satisfactory for the purpose, I found it desirable to attempt to 

 procure a ballistic galvanometer (preferably of the d'Arsonval type, to 

 avoid disturbances due to changes in outside magnetic fields) of period 

 so long that the throw of the coil due to a change of flux of the usual 

 sort, lasting for say thirty seconds, should not be sensibly different 



3 Beattie, Electrician, Dec, 1902; Peirce, These Proceedings, 43 (1907). 



4 G. Wiedemann, Galvanismus, 3, 738; Gumlich und Schmidt, Electrotech- 

 nische Zeitschrift, 21 (1900); Ruecker, Inaugural Dissertation, Halle, 1905; 

 A. Hoyt Taylor, Pays. Rev., 23 (1900). 



