MAGNETIC SHIELDING IN HOLLOW IRON CYLINDERS. 643 



terms of the order of field superposition. Of the two magnetising forces at right angles 

 to each other, let H x be the force first acting, H 2 the force superposed. Each force 

 acting alone produces the normal B-H induction curve. Let B x and B 2 be the two 

 components of the resultant induction in the directions of H x and H 2 respectively. 



When H 2 is superposed upon a pre-existing induction due to Hj, the B x component 

 always lies above the B 2 component. Repeated reversals of H 2 accentuate this result ; B x 

 is further increased, and concurrently with this B 2 is further lowered. For low fields, 

 the B x component is considerably above the normal induction curve, but as the fields 

 are increased a point (depending upon the relative strengths of both fields) is reached 

 where the curves cross, the B x component afterwards falling below the normal curve. 

 This first increase of B x at the early stages of induction is well marked and evidently 

 reaches a maximum in fig. xn. (CL and CLL, B c curves) ; while in fig. XL with a 

 lower value, and in fig. xiii. with a higher value of the superposed force, the effect is 

 in both cases somewhat reduced. 



Messrs Gerosa and Finzi (loc. cit.) have shown that an alternating current in an iron 

 wire increases, within due limits, the permeability of the iron to longitudinal magnetisa- 

 tion. The effect is regarded as clue to the violent agitation of the molecules brought 

 about by the rapid reversals of the circular magnetisation, due to the alternating 

 current in the wire, which corresponds to the reversals of the force H 2 in my ex- 

 periments. It must be noted, however, that repeated reversals of H 2 merely accentuate 

 the very considerable molecular disturbance caused by the first superposition of the 

 unidirectional force H 2 , in increasing the permeability of the iron at the earlier stages 

 of induction, to the constant force H x first acting. 



§ 20. Also the superposition of H 2 lowers the B 2 component below the normal 

 induction curve, with this exception, that at low values of H x the superposition of the 

 second force H 2 increases the B 2 component above the normal induction curve. This is, 

 however, a relatively small effect. The subsequent lowering of the B 2 component is one 

 of the more obvious effects of field superposition, and has been noted by Siemens, 

 Schultze, Messrs Evershed & Vignoles, Knott, Messrs Gerosa and Finzi and others 

 {see § 2). The first initial increase of B 2 , however, must be taken into account in stating 

 these results of field superposition. So far as these experiments are available, it follows 

 that for any two values of the force first acting (including Hi = 0) the curves measuring 

 the B 2 induction component cross, that for the lower value of E^ being at first below 

 and afterwards above that for the higher value of H x . 



§ 21. In this connection it is interesting to quote the remarks of Messrs Evershed 

 and Vignoles (loc. cit.) : "Starting with the idea of Mr Swinburne, that it should be 

 possible on the Weber h}'pothesis to get increased permeability in iron by magnetising 

 it first in one direction, and then in another at right angles to it, we proceeded to 

 investigate the point." They conclude as follows : — " Our results, in short, show, as far 

 as rhey go, (l) that no such increase of permeability as anticipated by Mr Swinburne 

 on the Weber hypothesis actually takes place ; and (2) that no increase of permeability in 



