632 MR JAMES RUSSELL ON 



Experimental investigations of the shielding ratio g have been made by Stefan and 

 Du Bois using the methods of oscillations and deflections. Dr Knott recently gave an 

 account before this Society, of exploring the field within a hollow iron sphere by compar- 

 ing the twist produced in a nickel wire per unit current and magnetised longitudinally 

 in various fields. Closely connected with this subject is the distribution of the pull 

 between copper conductors and the armatures, when these conductors are sunk in 

 tunnels. Mr Mordey * has measured the mechanical pull exerted by a conductor 

 sunk in a slotted armature, and Professor Du Bois and the writer the side thrust 

 experienced by iron rings or cylinders when shielding a magnetic field from con- 

 ductors. My measurements of this thrust, under various conditions of magnetisation, 

 led me to undertake this investigation. It may also be mentioned that iron shields 

 have been utilised in protecting delicate instruments from strong magnetic fields. Some 

 of Lord Kelvin's special types of galvanometers afford examples of this. 



§ 2. At an early stage of this investigation, and when dealing with forces acting 

 upon the iron in addition to that due to the transverse field, the subject of super- 

 posed magnetic inductions necessarily came to the front. Werner von Siemens, t 

 Schultze,| Kennelly,§ Messrs Evershed and Vignoles,§ Knott,|| Messrs Gerosa and 

 FiNZi,ir and Crook,** and doubtless others, have experimentally investigated this sub- 

 ject. In sections 14 to 29, the permeability of iron to magnetising forces at right 

 angles to each other will be dealt with independent of and uncomplicated by shielding 

 in hollow iron cylinders. 



I must here express my thanks to Dr Peddie for the ready advice given me at all 

 times during the course of these experiments. 



Objects of Investigation. 



§ 3. First. Magnetic Shielding. — The present contribution to this subject is 

 confined to the shielding which exists within thin hollow iron cylinders, when placed 

 in a uniform magnetic field, the lines of force of this field being at right angles to the 

 axis of the cylinder. 



I endeavour to determine experimentally the shielding ratio between the transverse 

 field and the field within the cylinder under the. following conditions, viz. : — 



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



transverse field. 



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



due to the transverse field. 



* Electrician, vol. xxxix. p. 190. 



t Scientific and Technical Payers of Werner von Siemens. English edition, vol. i. pp. 353 to 372 (1892). 

 X Wied. Ann., xxiv. p. 663 (1885). 



§ " On the Permeability of Iron at Right Angles to the Direction of Magnetisation," Electrician, vol. xxv. pp. 

 Ill, 141 and 158. || Proc. R.S.E., vol. xviii. p. 124. 



U Magnetic Induction in Iron, Ewing, 3rd ed., p. 330. ** American Journal of Science, vol. clxi. p. 365 (1901). 



