Peirce — Permeabilities and Reluctivities for Steel. 281 



observer, but the position of H was fixed. In setting up 

 apparatus of this sort, one should remember that if the distance 

 between B and P is not properly chosen, only a small round 

 portion of the image of the scale will be visible, not nearly 

 large enough to fill the aperture of B. 



The electromagnetic yoke used in the experiments described 

 here is represented in figure 3. The three exciting coils are 

 wound upon brass spools and these are mounted upon pieces 



Fig. 5. 

































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 1 











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Fig. 5. Curve showing the reluctivity of the soft Bessemer steel rod for 

 values of H between 3 and 160. 



of soft steel shafting carefully turned to fit the holes in the 

 heavy castings which complete the frame. The coils have 

 together 2956 turns and the wire of which they are made is 

 large enough to carry a current of 50 amperes for a few 

 minutes at a time without undue heating. The whole yoke 

 weighs about 300 kilograms. The apparatus is supplied with 

 jaws of a number of different forms, but for the results 

 recorded below conical jaws of the form shown in figure 4 

 with taper holes to receive the tapered ends of the specimen 

 to be tested were used. All the joints were made by Mr. 

 Thompson and seemed to be mechanically perfect. Before 

 each piece was tested the jaws were driven together with the 

 test-piece between, and then the whole was clamped. 



For very low excitations the field about the test-piece did 

 not seem to be quite uniform, but this difficulty disappeared 

 when the exciting current became as strong as one ampere. 



The intensities of the exciting currents were measured with 

 the help of a set of five Weston amperemeters of different 



