MAGNETIC SHIELDING IN HOLLOW IRON CYLINDERS. 633 



(c) When a longitudinal magnetising force is acting upon the iron in addition to 

 that due to the transverse field, 

 and to co-ordinate the results obtained with the magnetic induction in the iron under 

 the conditions. 1 also endeavour to show how far, and under what conditions, the 

 experimental results approximate to theoretical formulae which assume the permeability 

 to be uniform all round the shield and the absence of retentivity and coercive force in 

 the iron. 



§ 4. Second. Superposed Magnetic Inductions. — I also deal with the superposition 

 of two magnetising forces at right angles to each other, and the co-ordination of the 

 two components of the resultant magnetic induction under various conditions of 

 field superpositions. 



Third. Magnetic ceolotropy of demagnetised iron. — My experiments also show the 

 unequal directional permeability of iron after having been magnetised and then 

 demagnetised by decreasing reversals ; and a possible explanation is offered, based upon 

 the molecular theory of induction. 



Apparatus and Experimental Methods. 



§ 5. Two iron cylinders or shields were experimented with. They were made by 

 folding thin sheet iron round a roller, bevelling and overlapping the edges about one- 

 third cm. The edges were clamped together and soldered in position. The shields 

 were uniformly wound with wire, threading it in and out so that they could be 

 magnetised circularly, i.e., in a closed magnetic circuit. An exploring coil was also 

 threaded on, at the position relative to the transverse field shown in fig. in., to 

 enable determinations of the magnetic induction to be made. In addition to this, the 

 shields were also wound with four layers of wire (one layer only being shown in fig. ma.), 

 which formed an external solenoid, so that the iron could be magnetised longitudinally. 

 Also with a few turns of finer wire widely wound in a central position, and shown only 

 in fig. ma., so as to measure an average longitudinal induction in the iron extending 

 to about an inch on each side of the centres of the cylindrical shields, and corresponding 

 with the length of the rotating inductor within the shields to be immediately described. 



The dimensions of the shields are as follows :— 



Length, 



Outside ratius, li, 

 Thickness, d, 



Eatio, | 



R 



Shield A was made from transformer iron, and was heated to redness after being bent 

 into cylindrical form ; shield B from common tinned iron and not annealed in any 

 way. 



Shield A. 



Shield B. 



3 - 2 cms. 



13-2 cms 



1-435 „ 



1-45 „ 



0-0233 „ 



0-029 „ 



0-0162 „ 



0-02 „ 



