180 



Lord Rayleigh on the 



In the case of a very flattened planetoid (e = l), N becomes 

 in the limit equal to 47r. 



The force actually operative upon the iron is formed by 

 subtracting N-3 from that externally imposed, so that 



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

 we know the relation between § and -&, then the above equa- 

 tion gives us the relation between £>' and 3 for any proposed 

 ellipsoid of finite elongation. If we suppose that § is plotted 

 as a function of -3, we have only to add in the ordinates N 3, 

 proper to a straight line, in order to obtain the appropriate 

 curve for & f . 



As an example, let us apply this method to deduce the 

 behaviour of the soft iron of E wing's fig. 2, when made into 

 an ellipsoid whose polar axis is fifty times the equatorial axis, 

 and carried round a cycle through strong positive and strong 

 negative magnetism. We have 



N= ~ \log e 100-1} =4ttx -001442. 



The curve ABC (fig. 2), traced from Prof. E wing's, gives 



Fig. 2. 



the relation between § and 23, the latter of which we may 

 identify with 4^-3 *. The equation of the straight line is 



$ = NO =-001442 x4tt3; 



and with allowance for the different scales adopted for ordi- 

 nates and abscissas, is represented on the diagram by OD. 



* The curve is symmetrical with respect to O as centre, and § is 

 measured in C.G.S. units. 



