ELECTROMAGNETS. MAGNETISM OF IRON. 381 



23. Calculation of loss of power in iron due to hysteresis and 

 eddy currents. Hysteresis loss. The product of the energy loss 

 per cycle and the number of cycles per second gives the energy 

 lost per second, or, in other words, the power lost in the iron. 

 Therefore from equation (16) we have : 



P h = 77/F^ 1 - 6 x io- 7 (17) 



in which P h is the loss of power in watts in V cubic centimeters 

 of iron which is subjected to f magnetic cycles per second 

 through the range of the flux density cB. Values of 97 are 

 given in the table above. 



When a mass of iron is rotated in a magnetic field the direction 

 of magnetization is repeatedly reversed, but in a manner very 

 different from that in which the magnetization is reversed in a 

 stationary mass of iron by reversals of the magnetic field. Baily * 

 found for high flux densities a smaller hysteresis loss in a rotating 

 mass than in a stationary mass of iron for a given maximum flux 

 density, especially when the flux density is very large. Later 

 experiments by Dinaf show a smaller difference than that found 

 by Baily, and both Baily and Dina find the difference to be neg- 

 ligibly small for flux densities up to 16,000 lines per square cen- 

 timeter. 



Eddy current loss. The loss of energy by eddy currents has 

 no essential connection with the magnetic properties of the iron, 

 but the loss of power by eddy currents is usually considered in 

 conjunction with loss of power by hysteresis. The loss of power 

 by eddy currents is proportional to the volume of the iron, to the 

 square of the number of magnetic cycles per second, to the square 

 of the thickness of the laminations, and to the square of the 

 maximum flux density. Therefore 



P e = l<y 2 { 2 2 (i8)t 



in which P e is the eddy current loss in watts, V is the volume 



* Philosophical Transactions, clxxxvii, p. 7 I 5> 1896. 

 \Elek. Tech. Zeitschrift, 1902, p. 41. 



\ For a full discussion of this equation see Steinmetz, Alternating Current Phe- 

 nomena, third edition, pp. 129 to 149. 



