426 Scientific Proceedings, Royal Dublin Society. 



The component intensity is given by 



Ila = la s , _ ^ = I L (1 + KNcO^a) -g- -^- Sill a. 



I la = /z sin O 



1 + iVcos 2 a f K - —. — ^-r| — ^— ) 

 = /t sin a [1 + iVcos 2 a {K - I L /S')] approx. 

 The shape of the curve of initial magnetization shows that 



8' 



is negative, and writing it - A we have 



I L sin a - Iia. = ANI L sin o cos 2 a, 

 which gives a maximum at sin a = 0-576. 



It may be seen that the graphs in fig. 4 correspond generally to the 

 theory. The deviations for the spiral fields S' = 10 and S' = 20 from the 

 broken lines representing y = I L sin a follow the variations in sin a cos 2 a, 

 giving a maximum near sin a = 0'6 excepting at the ends where the 

 deviations are larger than are to be expected from the theory. 



The analysis, however, is important in showing how the demagnetizing 

 effect may be corrected. We have hitherto obtained a spiral field S' by 

 applying fields 



H' = 8' sin «, 

 F = S' cos a. 



The field H' is, however, diminished by the demagnetizing force by 



Nlia = NI L sin a approx., 



so that the actual spiral field S is somewhat less than S'. To obtain a 

 net spiral field S we have to apply a circular field F = S cos a and a 

 longitudinal field 



11' = S sin a + NIl sin a = (S + s) sin a, 



where s is a quantity to be determined from a knowledge of the demagnetizing 

 factor. 



Correction for the demagnetizing effect. — It will be shown later in the paper 

 that s = 2'5 compensates for the distorting effect of the demagnetizing effect 

 in the study of the twist of a spiral field S = 10, especially in the neigh- 

 bourhood of the pitch-angle 45°. The curve marked S = 10, fig. 4, shows how 

 the longitudinal intensity varies for the fields ; 



F= 10 cos a , 11= (10 + 2-5) sin «, 



