MAGNETIC SHIELDING IN HOLLOW IRON CYLINDERS. 667 



shielding ratio. Compare this with the curve obtained under the same conditions, but 

 with the circular field, H, being also equal to 64"25 units. The difference is marked. 



§ 56. These curves now fall to be compared with the theoretical shielding ratios. 

 The calculated value of H, being that due to the solenoid, and not the true magnetising 

 force in the iron, it was necessary to measure the longitudinal induction in the shields by 

 means of the exploring coil (§ 5). As mentioned at the end of § 15, the induction 

 measured in this way required correction. This was done graphically by means of data 

 determined experimentally. By taking a sufficient number of points on the induction 

 curves the same values of induction in the B-H curves of fig. iv. furnished corresponding 

 values of the theoretical ratios ((g)) and (g) from the dotted and dash curves respectively. 

 These values have been transferred to fig. xxxviii. (shield A). The circles are the values 

 obtained for the ((g)) ratios, and the crosses those obtained for the (g) ratios. It is at once 

 apparent that no simple relationship exists between the permeability (either dB/dB or 

 B/H) of the iron due to the longitudinal magnetising force and the experimentally 

 determined shielding ratios. The same remarks apply with equal force to shield B, 

 where the same general features hold. All the curves given for shield A are typical of 

 shield B. 



Summary of Conclusions. 

 I. Magnetic Shielding in Holloiv Iron Cylinders. 



(a) When no other magnetising force is acting upon the iron than that due to the 

 transverse field increased by increments from zero, the theoretical formula for the 

 shielding ratio (g) given in § 8 is very approximately fulfilled within the limits of the 

 transverse field used in the experimental determinations, viz., between H« = 4"4 and 130 

 C.G.S. units. 



The shielding ratio minus unity is proportional to what may be called the ratio 

 permeability B/H, and not to the differential permeability dB/dR. 



If, however, the transverse field is decreased from a maximum, theoretical formulae 

 are not applicable, the conditions as to absence of retentivity and coercive force not being 

 fulfilled. 



(b) When a circular magnetising force is acting upon the iron in addition to that due 

 to the transverse field, the order and manner in which the one field is superposed upon 

 the other affects the shielding ratio to an enormous extent. 



First. — When upon a pre-existing induction due to the transverse field increments 

 of the circular force ascending from zero to a maximum are superposed, the theoretical 

 formula for the shielding ratio ((g)) given in § 8 is fulfilled if the value of H f be not 

 unduly increased. The shielding ratio minus unity is proportional to the permeability 

 impressed upon the iron by the circular field ; m being defined as the differential 

 permeability dB c /dH c , and not the ratio permeability as was found to be the case where 

 the transverse field alone is acting upon the iron. 



