PEIRCE. — BEHAVIOR OF THE CORE OF AN ELECTROMAGNET. 117 



M 



In doing a small part of the work described below, I was able to use 

 either a Grassot Portable Fluxmeter, or a certain fixed laboratory 

 fiuxnieter (i* 7 ) furnished with a tall chimney to hold the 140 centi- 

 meter long fibre by which the coil was suspended. The cast-iron 

 magnet of this last mentioned instru- 

 ment had, when finished, the form 

 shown in plan in Figure 11 and was 

 45 mms. thick. The casting was 

 made with a web connecting the 

 poles, and this was removed after the 

 hole for the coil had been cut out 

 and finally reamed to a diameter of 

 exactly 5 cms. on a Browne and 

 Sharpe milling machine. The mag- 

 net was hardened and treated by Mr. 

 G. W. Thompson, the mechanician of 

 the Jefferson Physical Laboratory, 

 who has had much experience in 

 this kind of work. During the proc- 

 ess the poles were held in position 

 by an iron yoke. The core (shaded 

 in the diagram) within the coil is 

 41.3 mms. in outer diameter, and is 

 about 7 mms. thick. The instru- 

 ment was constructed and set up 

 by Mr. John Coulson, who has 

 helped me in countless ways during 

 the progress of the work. It was 

 comparatively easy to substitute one of the set of coils belonging to 

 this fluxmeter for another. For certain purposes it was convenient to 

 have a coil of 200 turns of stout insulated wire which was wound about 

 the magnet, though the latter had a large permanent moment. 



Figure 11. 



Plan of one of the permanent 

 magnets of the fluxmeter F; the 

 shaded area represents the cross-sec- 

 tion of the soft iron core. 



The Coefficients of Self-Induction of a Circuit which 



has an Iron Core. 



"When many years ago it was found that the induction B at a given 

 point in a piece of iron exposed to a given magnetic field H is not only 

 not in general proportional to the intensity of the exciting force, but is 

 not even determined when H is given, it became evident that no such 

 constant can exist in the case of an inductive circuit which "contains" 

 a magnetic metal as was assumed in the conception of Neumann's 



