464 PROFESSOK KNOTT ON THE STRAINS PRODUCE!) IN IRON, 



But the differences that exist seem of comparatively small importance beside 

 the general agreement, even to numerical details, among the dilatations of the different 

 types of nickel tube. 



The results for the cobalt tube call for little remark. The linear dilatations are 

 much smaller than for nickel, as a glance at the curves on Plate I. shows. The cubical 

 dilatation of the metal is, however, greater, being appreciable enough to be measured. 



The broad difference between the two metals is that the molecular groups yield 

 more readily to the magnetizing force in nickel than in cobalt. The nickel 

 curves all show an approximately saturated condition in the substance ; but there is 

 no evidence of such a condition being approached in the case of cobalt. 



^ 6. The Bored Iron and Steel Tubes. — The results for iron are, as compared 

 with those for nickel, of a very complex character. 



Consider, first of all, the volume changes of bore in the various sets of tubes as 

 given in the column headed Bv in Tables VI., VIII., IX. A gradual change in the 

 behaviour of the tube as the bore is made larger is very apparent in the case of 

 Tubes A and B (Table VI. and Plate II., first and third rows of graphs). Also 

 there is an evident parallelism in the two series A and B. Thus, in I. and II. of both 

 sets, the volume of bore diminishes steadily as the field increases. In III., however, 

 the change of volume passes through a curious minimum distinctly shown in the 

 graphs. In IV. and V. this minimum becomes more evident, the negative change 

 of volume diminishing markedly in the higher fields. Finally, in VI. and VII. this 

 diminishing negative change becomes an increasing positive change. In the A set 

 a particular peculiarity is associated with a field which is lower than that associated 

 with the corresponding peculiarity in the B set. For example, the fields associated 

 with the minimum volume change or greatest diminution of volume are : — 



In A III. 105 and 260 in B III. ; 



„ A IV. 90 „ 200 „ B IV. ; 



„ A V. 70 „ 165 „ B V. ; 



„ A VI. 40 „ 90 „ B VI. ; 



„ A VII. 20 „ 40 „ B VII. ; 



while the change of sign from negative to positive volume change occurs in fields 



70, 35, 140, and 75 



in A VI., A VI 1., B VI., and B VII. respectively. 



A similar parallelism is shown in the two sets of results for the changes in external 

 volume of the plugged tubes. Compare, for example, the oV curves in the first and 

 third rows of Plate II. 



The same feature is reproduced in the columns headed A, the measurements, 

 namely, of the linear dilatation in the direction of the magnetizing force. 

 A glance at the curves figured in the second and fourth rows of Plate II. shows 



