MAGNETIC SHIELDING IN HOLLOW IRON CYLINDERS. 665 



We may therefore conclude that repeated reversals of the circular field superposed 

 upon a pre-existing induction due to the transverse field (TCC conditions) abnormally 

 increases the shielding ratio, at the period of maximum permeability due to H c , but 

 that after this period is passed the shielding ratio, minus unity, rapidly becomes 

 proportional to the values of dB c /dH. c , as was found to be the case under the TC 

 conditions (§ 48). 



Conclusions under Cyclic conditions. 



§ 52. Table XIX. supplies the data for both shields when, upon a pre-existing 

 induction due to the transverse field of H t =20*9 C.G.S. units, the superposed circular 

 field carries the iron round a complete magnetic cycle. The circular field, after being 

 repeatedly reversed, starts at a positive maximum, is reduced by steps to zero, increased 

 by steps to a negative maximum, again reduced to zero, and finally increased to a 

 positive maximum. At each step the shielded field is measured by means of the 

 rotating inductor. The first columns give the average readings of H c between positive 

 and negative maxima, and between negative and positive maxima, the second and 

 third columns the corresponding values of the weakened fields within the cylinders and 

 the shielding ratios respectively. The values of the shielding ratios are plotted as 

 dash line curves in fig. vn., and their close connection with the theoretical curves ((g)) 

 (fig. vii.) obtained from the corresponding values of dB/dH. (Table II.) impressed 

 upon the iron by the circular field is at once apparent. The theoretical curves take 

 very different values at their extreme positions, according as the B-H curve is 

 descending from or ascending to the maximum values of B. This difference the 

 experimentally determined ratio curves (g', fig. vn.) necessarily fail to show, and 

 comparison may also be made between Tables XIX. and II. on this point. The 

 theoretical values ((g)) given where B has ascended to a maximum correspond very 

 closely with the experimentally determined ratios, being practically values obtained 

 under the TCC conditions. The high maximum values of the shielding ratios obtained 

 at the maximum values of dB c /dH c are very marked. For shield A the theoretical 

 maximum value is 97, the experimentally determinal maximum value 500, when in 

 both cases H c = —1*2 C.G.S. units. For shield B, the theoretical maximum value is 

 61, the experimentally determined maximum value 131, when in both cases H c = — 2'3 

 C.G.S. units. These cycles in reality show the change of the shielding ratio that takes 

 place as the circular field is varied between its extreme positions under the TCC 

 conditions. We may therefore conclude that an alternating circular force superposed 

 upon a pre-existing induction due to the transverse field, abnormally increases the per- 

 meability of the iron to the constant force H t , so that the shielding ratio minus unity is 

 greatly in excess even of the high values of dB c /dIi c which occur during the superposed 

 cyclic process ; but that, as H c ascends to its positive or negative cyclic extremes, the 

 shielding ratio approximates to the values which obtain under the TCC conditions. 



