MAGNETIC SHIELDING IN HOLLOW IRON CYLINDERS. 657 



transverse (TO conditions) are more than double the shielding ratios which obtain 

 when the transverse is superimposed upon the circular (TC conditions) ; and the 

 maximum shielding ratios under the TCC conditions are as much as six or seven times 

 as great as the shielding; ratios which obtain for the same field values under the CTT 

 conditions. Finally, in whatever way the two magnetic fields are superimposed the one 

 upon the other, all the shielding ratio curves seem to approximate, as the circular 

 magnetising force is further and further increased to the same minimum asymptotic 

 value. Obviously the shielding ratio curves might be plotted for various fixed values 

 of H c against values of H, as abscissae. In figs, xxxvi. and xxxvu.* this has been done 

 for shield A under two conditions of field superposition, viz., TC and CTT conditions. 

 The curve where H c =0 is the same for both figs. 



Co-ordination of Shielding Ratios with Magnetic Induction. 



§ 43. The question may now be introduced whether the very large differences in the 

 values of the shielding ratios, under the various conditions, can in any way be co- 

 ordinated with the magnetic induction in the iron shields. Reference to fig. in. shows 

 that when both the magnetising forces H { and H c act upon the iron, the induction will be a 

 maximum on one side of the shield and a minimum on the other. In the figure, the left 

 side, m, is that of maximum induction, and the right side, n, that of minimum induction. 



Table XVIII. gives the inductions at positions m and n of the shields A and B when 

 increments of the circular field are superimposed upon a pre-existing induction due to 

 the transverse field (TC condition) ; distinct sets of experiments being made for two 

 values of the latter, viz., H, = 4"37 (not tabulated) and H,, = 20'9 C.G.S. units. In the 

 third and sixth columns the measurements are in scale divisions of the ballistic galvan- 

 ometer. In figs, xxviii. and xxix. the values of B at these positions of maximum and 

 minimum inductions are plotted as ordinates against the corresponding values of H c 

 as abscissae, for shields A and B respectively. The curves m and n are the induction 

 curves at positions m and n, fig. III., when H e =20'9; and the dash line curves the 

 induction curves at same positions when H t = 4'37 C.G.S. units. 



Table XVII., on the other hand, gives the inductions at positions m and n of the 

 shields A and B, when upon a pre-existing induction due to the circular field between 

 the limits of H c = and H c =9 C.G.S. units, the transverse field is superimposed and 

 then repeatedly reversed. The first superposition of H f corresponds to the CT 

 conditions, the repeated reversals of H, to the CTT conditions, separate sets of 

 experiments being made for the two values of the transverse field given above. In 

 figs. xxx. and xxxi. # the final values of B (and therefore the inductions under the 

 CTT conditions) at the positions of maximum and minimum inductions are plotted 

 against the corresponding values of the circular field for shields A and B respectively. 

 As before, the m and n curves are the induction curves at positions m and n, fig. in., 



* In fig. xxxvu. " shield B ; ' should read " shield A " ; and in fig. xxxi. " shield A " should read " shield B." See 

 Plate 5. 



