Energy of Magnetized Iron. 179 



this paper should be noted, as showing the enormous advantage 

 which a ring-shaped core, or core forming a complete mag- 

 netic circuit, possesses over a short bar-core with ends. In 

 an ordinary induction-coil, so long as the current in the 

 primary circuit is merely made and broken, a short core is 

 necessary, since a ring-core would lose but a small percentage 

 of its magnetism at each brake, but where reversal of the 

 magnetizing takes place, a core approximating to the con- 

 dition of endlessness has an advantage in respect of power 

 which fig. 3 makes obvious." I confess that I do not follow 

 this. It seems to me, on the contrary, that a closed magnetic 

 circuit is above all things to be avoided, as leading to waste of 

 the greater part of the power transferred. 



A like objection applies to the use of a closed electro- 

 magnet as a " throttle " in an alternate-current circuit. 



When we know, as from Prof. Ewing's results, the be- 

 haviour of a given sample of iron under the influence of 

 various forces § actually operative, we can deduce by means of 

 Poisson's theory the magnetism assumed by ellipsoids of any 

 shape in response to any uniform external force §'. If -3 be 

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

 demagnetizing effect of -3 1 is N-3, where N is a numerical 

 constant, a function of the eccentricity (e) *. When the 

 ellipsoid is of the ovary or elongated form, 



a = b= x / (l — e 2 )c, 



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



JN- 3 , 

 and at the other extreme of elongation assuming the form 



N=4^ 2 (log 2 i-l). 

 If the ellipsoid is of the planetary form, 



and 



* Maxwell's < Electricity and Magnetism,' § 438. 



t There is here a slight variation from Maxwell's notation. 



N2 



1 



