IRON, STEEL, AND NICKEL TUBES IN THE MAGNETIC FIELD. 553 



No. I. may receive a sufficiently plausible explanation. With the nickel tubes there is 

 no change of sign of dilatation in the higher fields. This would seem to imply that the 

 magnetic strains produced in a nickel bar are of a comparatively simple character. 

 Further discussion of these interesting points must be deferred until direct measure- 

 ments of elongation have been obtained with the various tubes under the same magnetic 

 influences. 



The broad results established by these experiments may be summarised as 

 follows : — 



1. The internal capacity of tubes of iron, steel, and nickel, is appreciably altered 

 in strong magnetic fields. 



2. Broadly speaking, the tendency is for the iron and steel tubes to show decrease 

 of volume in low fields and increase of volume in moderate and high fields, except 

 with tubes of small bore, where the volume is diminished in high fields also. Other 

 exceptional instances, especially in the case of steel, seem referable to heterogeneity of 

 structure or unequal tempering. 



3. On the other hand, the nickel tubes show increase of volume in very low fields 

 and large decrease of volume in all other fields. 



4. The greatest decrease of volume in the nickel tubes is nearly 10 times the 

 greatest increase of volume in the iron tubes. 



5. The phenomenon of hysteresis is very conspicuous in the march of volume 

 change with magnetic force (Plate VI.). The effects, however, are too complicated 

 to be briefly summarised (see § 12). 



6. The great diversity in the behaviour of certain tubes, cut originally from the 

 same bar and bored out to exactly the same extent, shows that the character of the strain 

 accompanying a given magnetisation is profoundly influenced by differences of structure 

 and temper, which have no appreciable influence upon other magnetic qualities. 



7. As a rule, the wider the bore of the tube the greater the maximum change of 

 volume. In tubes of fairly wide bores the dilatations in strong fields seem to tend to 

 equality. 



8. The behaviour of any chosen tube in fields of different distribution shows that 

 the strain is of a complicated character. This and other considerations suggest that 

 each tube, when powerfully magnetised, becomes shaped either like a spindle or like an 

 hour-glass, and that the closing of the end of the tube by means of a cap has an 

 appreciable influence on the magnitude of the strain, as measured by the volume change. 



9. The average dilatations are of the order 10" 6 in the case of iron, and 10" 5 in 

 the case of nickel. 



