PEIRCE. — MAGNETS OF HARDENED CAST IRON, 715 



current Strengths the induction flux in the air between tlie poles; the 

 ordiuates in Figure G represent, in hundreds of lines per square centi- 

 metre, the induction across the centre of the gap X Y, while the abscis- 

 sas represent the current in the coils measured in amperes; the R-curve 

 belongs to the soft and the N-curve to the supposed hard casting. In a 

 second experiment made with the same castings, one of the coils on each 

 was used as a primary and the other, which was connected with a bal- 

 listic galvanometer, as a secondary ; this procedure gave the curves of 

 Figure 7, which show clea.rly that the second casting, which was very 

 hard to the file on the surface, was still soft inside. The diagrams of 

 Figure t?, drawn on a different scale, give the results of a similar exper- 

 iment on the soft casting just mentioned and a third ciiilled casting of 

 the same form. These striking curves, which were reduced 'from a large 

 drawing, represent the observations accurately, and illustrate the fact 

 that it is possible to make the whole inside of a massive casting, like the 

 one here used, magnetically very hard. 



When the gap X Y of this third casting had been closed by a piece of 

 soft iron, and a heavy current had been sent through the coils for a few 

 moments, in such a way as to make one of the jirojections a north pole 

 and the other a south pole, the casting became a fairly strong permanent 

 magnet : the flux per square centimetre across the middle of the air gap 

 was fnially 285. The flux per square centimetre across the gap of a sim- 

 ilar, though considerably less heavy, hardened cast-iron magnet, belonging 

 to one of a number of excellent d'Arsonval galvanometers furnished by 

 Messrs. Leeds and Northrop, is 187. In this magnet the pole projec- 

 tions are a little shorter and the gap a trifle wider than in the castings 4 

 described above. 



Jefferson Physical Laboratoet, 



Hakvari) University, 



Cambridge, Mass. 



