the Magnetic Hysteresis of Cobalt, 275 



mechanical work required to effect the complete magnetic 

 reversal of a crowd of small compass-needles is proportional 

 to the l*6th power of the aggregate maximum magnetic 

 moment before or after completion of the cycle. 



The observations also furnished a simple magnetization- 

 curve for the metal (fig. 4) and a series of values of the 

 permeability stated in terms of the flux-density B and of the 

 magnetic force H (fig. 5). 



A study of these curves shows that this cast cobalt magne- 

 tically resembles many ordinary qualities of cast iron. 



As a comparison, a series of similar observations were 

 taken on a circular cast-iron ring. 



This ring was prepared for us by Messrs. Easton, Anderson, 

 and Goolden from a variety of cast iron used by them for 

 dynamo purposes. The transverse section of the ring was 

 rectangular. 



The dimensions of the ring were as follows : — 



Mean outside diameter % . 5*000 inch 



Mean inside diameter . . . 4*000 „ 



Depth of ring 0*500 „ 



Mean thickness 1*000 „ 



Mass of ring 1*8086 lbs. 



Specific gravity 7*042 



The ring was insulated and wound over with two primary 

 coils of No. 20 s.w.G. cotton-covered copper wire, the inner 

 layer having 248 turns and the outer 243 turns. 



The primary coil was overlaid with two secondary coils of 

 JSo. 36 silk-covered copper wire, one having 200 turns and 

 the other 20 turns. The ring was submitted to the process 

 above described for obtaining the cyclic magnetization-curves 

 by determining with the ballistic galvanometer the value of B 

 corresponding to various cyclic values of H. These obser- 

 vations were taken by Mr. H. I. Lewenz in January 1898, 

 in the Pender Electrical Laboratory, University College, 

 London. 



The hysteresis loops were then carefully plotted out (fig. 6) 

 by the same observer. From the measurements of area a 

 Steinmetz curve was drawn (fig. 7), the ordinates of which 

 represent ergs per cubic centimetre per cycle and the abscissae 

 maximum flux-density during the cycle. Also a curve (fig. 8) 

 was drawn, the abscissae and ordinates of which were ths 

 ordinary logarithms of the above quantities. 



This latter curve proves to be a straight line over a consi- 

 derable portion of its length. The inclination of the iins, 

 however, shows that the hysteretic exponent for this cast ircn 



X2 



