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PROCEEDINGS OF THE AMERICAN ACADEMY. 



of Cambridgeport, Mass., where we have obtained during the last few 

 years a large number of castings of different forms for permanent 

 magnets which proved when made and seasoned to be very strong and 

 to have remarkably small temperature coefficients. 



It will be noticed that this iron while soft is rather more permeable 

 than that which was the foundation for the formula for reluctivity in 

 " Ordinary Dynamo Cast Iron " given by Messrs. Houston and Ken- 

 nelly in their Electro-Dynamic Machinery, but is very similar so far as 



Figure 3. 



results are available with the standard " Gray Cast Iron " used for the 

 table given in the pamphlet on the "Magnetic Circuit" of the Inter- 

 national Textbook Company. Although I had at command a much 

 larger yoke than the one used, no attempt was made to carry the exci- 

 tation beyond 15,000 gausses. The ultimate value of I in my hardened 

 cast iron was about the same as that which Ewing gives for " Cast Iron " 

 in " Magnetic Induction in Iron and Other Metals," § 93. 



The magnetic effects of hardening upon a mass of cast iron are often 

 very noticeable at comparatively low excitations. The two halves of 

 each of two thick castings, one soft, the other very hard, of the form 

 shown in Figure 3, were wound with 156 turns each of insulated wire, 

 and the two coils on each casting were so connected in series that when 

 a current was sent through the circuit both conspired to make one of 

 the projections (say X) a north pole and the other (Y) a south pole. 

 With each of the castings a rude kind of hysteresis diagram was ob- 

 tained by measuring for different current strengths the values of the 

 induction flux across a definite area in the air gap between the poles. 

 These fluxes plotted against the corresponding currents gave the dia- 

 grams shown in Figure 4. The A curve belongs to the soft casting, the 



