654 MB. JAMES RUSSELL ON 



demagnetising by reversals, it appears to the writer that iron demagnetised in this way 

 is bound to exhibit aeolotropic properties at the early stages of induction. 



I. (b) Shielding Ratios with Transverse and Circular Fields. 



§ 38. The addition of the circular magnetising force H c supplies a nearer approach 

 to uniform magnetisation than when the transverse field is the only force acting upon 

 the iron, more especially when the values of H ( are small. We have consequently a 

 nearer approach to uniform values of B/H or dB/dJl all round the shield, one of the 

 conditions assumed in the theoretical shielding formulae. 



When the circular field is increased from zero, preliminary experiments showed 

 that the order and manner in which the one field is superimposed upon the other 

 affected the shielding ratio to an enormous extent. * The following sequences were 

 tried, the value of the transverse field being in all cases equal to 20*9 C.G.S. units. 



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



field were superposed increments of the circular field ascend- i TC. 



ing from zero to a maximum. I 



Second. — When upon a pre-existing induction due to the transverse] 



field were superposed repeated reversals of the circular field I TCC. 



ascending from zero to a maximum. ) 



Third. — When upon a pre-existing induction due to the circular') 



field ascending from zero to a maximum was superposed atl CT. 



each increment the (uni-directional) transverse field. J 



Fourth. — When upon a pre-existing induction due to the circular \ 



field ascending from zero to a maximum were superposed atl CTT. 



each increment repeated reversals of the transverse field. j 



Fifth. — When upon a pre-existing induction due to the transverse \ 



field, the superposed circular field carries the iron in steps L Cyclic, 



round a complete magnetic cycle or series of cycles. j 



Let the above first four sequences of superposition of magnetic fields be referred to, 

 for the sake of brevity, by the letters to their right, T and C signifying transverse and 

 circular fields respectively, and also the order in which they are impressed on the iron 

 shields. CC and TT mean that these fields are repeatedly reversed. Further, let these 

 symbols be used to indicate not only the conditions themselves, but also the shielding 

 ratio curves obtained under these conditions. 



§ 39. Figs. xx. and xxi. show the shielding ratio curves obtained under the above 



* It is of interest to observe that it has also been noted that the amount of twist produced in an iron wire by a 

 given combination of circular and longitudinal magnetisms, depends largely upon the order in which these are 

 applied. See " Magnetism and Twist in Iron and Nickel," by Dr Knott, Trans. R.S.E., vol. xxxvi. p. 490, 1891. 



