584 Lord Rayleigh on the Induction- Coil, 



the iron may deviate widely from that described as ideal, and 

 the quantity denoted by L has no existence. Bat the 

 principle remains that the energy of charge at the moment 

 preceding the secondary spark cannot exceed, though it may 

 somewhat closely approach, the energy of the primary 

 current before break. 



We have next to consider how the energy of the primary 

 current is to be reckoned, and here we encounter questions as 

 to which opinion is not yet undivided. The general opinion 

 would, I suppose, be that the bodily magnetization of the iron 

 represents a large store of available energy. If this be 

 correct, the inference would be irresistible in favour of a 

 very long, or a completely closed, iron core. Some years 

 ago *, reasoning on the basis of the theory of Warburg and 

 Hopkinson, I endeavoured to show that highly magnetized 

 iron could not be regarded as a store of energy — that the 

 energy expended in producing the magnetization was re- 

 coverable but to a small extent, or not at all. Although this 

 conclusion does not appear to have been accepted, perhaps in 

 consequence of an erroneous application to alternating current 

 transformers, I still see no means of escape from it. The 

 available energy of a highly magnetized closed circuit of iron 

 is insignificant. If the length be limited, there is available 

 energy, in virtue of the free polarity at the ends. 



The theory is best illustrated by the case of an ellipsoid of 

 revolution exposed to uniform external magnetizing force 

 <§' acting parallel to the axis. "If 3 be the magnetization 

 parallel to the axis of symmetry (2c), the demagnetizing 

 effect of 3 is N3, where N is a numerical constant, a function 

 of the eccentricity (<?) . When the ellipsoid is of the ovary or 

 elongated form, 



a = b = c y/(l—e q ), 



becoming in the limiting case of the sphere (e = 0), 



and at the other extreme of elongation assuming the form 



N=4^(logf-l) (5) 



" The force actually operative upon the iron is found by 



* " On the Energy of Magnetized Iron," Phil. Mag. xxii. p. 175 (1886) ; 

 Scientific Papers, ii. p. 543. 



