Lord Rayleigh on tJie Induction- Coil. 585 



subtracting N3 from that externally imposed, so that 



and if from experiments on very elongated ellipsoids (N = 0) 

 we know the relation between JQ and 3, then the above 

 equation gives us the relation between «£/ and 3 for any 

 proposed ellipsoid of moderate elongation. If we suppose 

 that «jp is plotted as a function of 3, we have only to add in 

 the ordinates N3, proper to a straight line, in order to obtain 

 the appropriate curve for «£)'." 



The work expended in magnetizing the iron is per unit of 

 volume 



if we reckon from the condition of zero magnetization. The 

 first part is practically wasted; the second, which in most 

 cases of open magnetic circuits is much the larger, is com- 

 pletely recovered when the iron is demagnetized. 



If it appear paradoxical that the large integral electro- 

 motive force which would accompany the disappearance of 

 high magnetization in a closed iron circuit should be so 

 inefficient, we must remember that the mechanical value of 

 electromotive force depends upon the magnitude of the 

 current which it drives, and that in the present case the 

 existence of more than a very small current is inconsistent 

 with that drop of magnetization upon which the electromotive 

 force depends. 



The considerations above explained are of interest in the 

 present question as affording a limit depending only upon 

 the iron core and the secondary capacity. For 3 cannot 

 exceed a value estimated at about 1700 c.G.s., whatever may 

 be the magnetizing force of the primary current. Thus if v 

 be the volume of the core, the maximum energy * is 



^NxvxHOO 2 ; 



and the limit to V is found by equating this to J^V 2 , so that 



V=1700 v'(Nt>/?) (6) 



I have made a rough application of this formula to a coil 

 in my possession, with results that may be here recorded. 

 The core had a diameter of 3 cm. and a length of 27 cm., so 

 that v=180 c. c. From (5), properly applicable only to an 

 ellipsoid, we get by setting "2a = 3, 2c = 27, N = *30. 



The capacity of the secondary is more difficult to deal with. 



* The energy of the primary current without a core is here neglected. 



