1887.] On the Magnetisation of Iron in Strong Fields, 207 



also tested by drawing it out of the field. The results were in close 

 agreement with those given above for the other sample. 



In fig. 3 the general results for Lowmoor wrought iron (Sample B) 

 and cast iron are shown by carves which give the relation (1) of the 

 induction ^ within the metal neck to the current in the field magnet 

 coils, and (2) of the induction or magnetic force in the space im- 

 mediately surrounding the neck to the current in the field magnet 

 coils. The full lines are for the Lowmoor forging, and the broken 

 lines are for cast iron. The field produced by a given current is (at 

 its higher values) rather less strong in the case of cast iron, probably 

 because the larger size of the cast iron neck allowed a greater portion 

 of the whole induction from pole to pole to find its way through the 

 metal. (Compare X x for cast iron and for Lowmoor.) 



The magnetic force within the metal (J) differs from the field in 

 the surrounding space by an amount which cannot be estimated 

 without a knowledge of the distribution of free magnetism on the 

 pole-pieces and conical faces of the bobbin. It appears probable that 

 with the dimensions of the various parts used in these experiments, 

 the magnetic force within the metal is less, but not very greatly less, 

 than the outside and closely neighbouring field. In the absence of 

 any exact knowledge of Jj, it is interesting to examine the relation of 

 1| to the outside field. Thus, (g— outside field)/47r gives a quantity 

 which is probably not much less than the intensity of magnetism |j. 

 The values of this quantity for Lowmoor wrought iron, Swedish 

 wrought iron, and cast iron are stated below. In the case of the 

 Swedish iron the values of |j — g r given in the previous table for that 

 metal have had 500 added to allow for the residual magnetism. 

 Again, the quantity J) /outside field is probably not much less than 

 the magnetic permeability fx, : its values also are given below. 



I. Lowmoor "Wrought Iron. 



Outside field. 



ft 



|8 — outside field 



i 



4tt. 



outside field. 



3,630 



24,700 



1680 



6-80 



6,680 



27,610 



1670 



4-13 



7,800 



28,870 



1680 



3-70 



8,810 



29,350 



1630 



333 



9,500 



30,200 



1650 



3-18 



9,780 



30,680 



1660 



3-14 



10,360 



30,830 



1630 



2-98 



10,840 



31,370 



1630 



2-89 



11,180 



31,560 



1620 



2-82 



