212 MR. S. BIDWELL ON CHANGES PRODUCED BY MAGNETISATION IN 



Fig. 3 



Upon examination of these curves it at once appears that the general character of 

 the phenomenon under discussion is just the same in rings as it is in rods. In both 

 cases a continually increasing magnetising force is accompanied by elongation, which 

 reaches a maximum, then falls to zero, and ultimately becomes negative. And both 

 sets of curves appear to follow a similar law, ascending in a form suggestive of a 

 parabola, descending in a straight line. Greater differences might easily be found to 

 occur in rings of different specimens of iron than those which exist between the rings 

 and the rod in the present instance. The maximum elongation (23 ten-millionths) of 

 the rod which happened to be chosen for these experiments is indeed less than that of 

 any of the rings, and its retraction begins at an earlier stage. This, no doubt, is 

 partly owing to the effect of the ends and to want of uniformity in the magnetisation, 

 some portions of the rod having attained the state of maximum extension while others 

 had either not yet reached it, or had passed it. But it may also be to a great extent 

 due merely to the quality of the iron, for I have myself observed in an iron rod a 

 maximum extension of as much as 45 ten-millionths* (equal to the maximum extension 

 of ring No. and greater than that of ring No. 1), while an elongation of 56 ten- 

 millionths (greater than that of any of the rings) has been recorded by JouLE.t On 

 the other hand, the maximum extensions of the rings would undoubtedly have been 

 less if they had been made of harder iron, instead of the softest which was con- 

 veniently procurable. 



* ' Roy. Soc. Proc.,' vol. 40, 1886, p. 117. 



t See JOULE'S paper, ' Phil. Mag.,' vol. 30, 1847, p. 76, or the Physical Society's ' Reprint,' p. 235. 

 The greatest elongation recorded occurs in Exp. 2, p. 240. It amounted to 28 scale divisions, each of 

 which corresponded to an extension of 1/138528 inch in a rod 36 inches long. 



28/36 x 138528 = 0-0000056 

 or 56 teu-millionths of the length of the rod. 



