MAGNETIC SHIELDING IN HOLLOW IRON CYLINDERS. 663 



reversals of the transverse field. The difference in the number of magnetic lines converging 

 upon the iron appears, however, to point to the conclusion that the permeability of the 

 shields is somewhat decreased to the force which is not constant, i.e., to the transverse 

 field. We must, however, look for an explanation of the great diminution in the 

 values of the shielding ratio to the effects following upon the differences of field super- 

 position as compared with what we have called the normal or TC conditions. Under 

 these conditions the pre-existing induction due to H, is lightly held, the demagnetising 

 factor being, as we have seen, very large. But under the CTT conditions now under 

 consideration, the circular induction, being first in possession of and forming a closed 

 magnetic circuit on the iron, is rigidly held. 



The superposed repeated reversals of the transverse field have a double effect : (first) 

 the circular magnetisation is increased as a result of molecular vibration (§ 46) ; and 

 (second) the induction due to the transverse field, determined principally by the geo- 

 metrical factor (§ 47), ousts from their position in the m side of the shield the circular 

 magnetic lines, abnormally increased during reversals of H,, and compels them to com- 

 plete their circuit across the central shielded space. In other words, the shielding 

 effect is reduced. Diagrams (1) and (2), fig. xxxv., are intended to graphically 

 represent this result. (1) shows the shield circularly magnetised by the force H c , the 

 heavy circular line representing any given number of magnetic lines. The super- 

 position of repeated reversals of the transverse field increases the circular induction (a 

 vibration effect), and (2) shows that four of them are compelled to cross the shielded 

 space, thus lowering the shielding ratio in comparison with diagram (l), fig. xxxiv., 

 where the transverse field alone is acting, or in comparison with diagrams (2), (3) and 

 (4), fig. xxxiv., where the transverse field is that first acting, and H c the force superposed. 

 The conclusion therefore is that when repeated reversals of the transverse field are 

 superposed upon a pre-existing induction due to the circular magnetising force (CTT 

 conditions), the permeability of the iron is increased to the force which remains 

 constant, viz., the circular field, and reduced to the force which is being reversed, viz., 

 the transverse field. Hence the shielding ratio also is reduced, and does not bear any 

 simple relationship to the values either of B/H or dB/dH. due to the circular force ; but 

 is a complicated function of the magnetic properties of the iron, embracing permeability, 

 the phenomena associated with hysteresis, and the effects of molecular vibration, all as 

 determined by the particular mode of field superposition under the CTT conditions. 



Conclusions under CT conditions. 



§ 50. The shielding ratio curves obtained when the transverse field is simply 



superposed upon a pre-existing induction due to H c (CT conditions), fig. xxiv. 



(shield A) and fig. xxv. (shield B), lie between those obtained under the TC and 



CTT conditions. They more nearly resemble the former, but approximate more 



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



