MAGNETIC SHIELDING- IN HOLLOW IRON CYLINDERS. 639 



ing field at the same value, the field within the shield is nearly 4 C.G.S. units. In 

 both cases the residual fields, after H f has been withdrawn, do not differ greatly from 

 each other, being 1*4 and 1*9 C.G.S. units respectively. With a decreasing field less 

 than 50, (H) w became negative, while between the limits of H ( = 20 and H, — 0, (H)„, is 

 equal to (H) r in magnitude and sign. The same curves for shield A (fig. vin.) show 

 the same peculiarities, the lower values of the full line curve (H)„, denoting better 

 shielding, or a higher permeability ; and the lower values of the dotted line curves (H) r 

 denoting less coercive force. If the g ratios (H),/(H) W be considered, the shielding 

 becomes infinite when H, = 50and 55 C.G.S. units for shields B and A respectively, 

 and when H t is further decreased, the ratio between the transverse field and the shielded 

 field becomes negative. If, on the other hand, the residual field be taken into account, 

 and the g" ratio 



H, 



(H)„-{-(H) ( . 



be considered, the shielding ratios increase very rapidly, as H, is decreased and becomes 

 infinite between the limits of H, = 20 and H, = 0. But it must be noted that the g" 

 ratios never become negative. It is obvious that any comparison with the theoretical 

 ratios is debarred, the theoretical conditions as to absence of retentivity and coercive 

 force not being fulfilled. 



§ 13. Before leaving this subject attention may be called to the third (H), columns 

 Table VI. (figs. vin. and ix.). It will be noted that for both shields the residual field 

 (H),. is less when the transverse field is reduced to zero at one step, than when it is 

 reduced by a series of steps to zero. Although the difference is not large it is sufficiently 

 well marked, and illustrates the known fact that sudden withdrawal of the magnetising 

 force produces the greater internal commotion and so lessens residual magnetisation in 

 the same way that vibration would. 



Table IV. shows the ratio which exists between the transverse field, disturbed H/ 

 and undisturbed H, by the presence of the iron shield for various values of the 

 transverse field. (See § 47). 



II. Superposed Magnetic Inductions in Iron. — (a) Magnetising Forces 



increasing from Zero. 



§14. At a later stage of this investigation it was found that the value of the 

 transverse field being equal to 20"9 C.G.S. units, the shielding ratio was 12 per cent, 

 higher when the shields had been previously demagnetised by decreasing reversals of 

 the circular field, than when they had been previously demagnetised by decreasing 

 reversals of the longitudinal field.* By way of elucidating this point, various 

 experiments were made ; and these fields being at right angles to one another, the 



* See § 54 ; also §§ 30-37 on Magnetic ^Eolotropy of Demagnetised Iron. 

 TRANS. ROY. SOC. EDIN., VOL. XL. PART III. (NO. 26). 5 b 



