1894.] Dimensions of Wires and Rings of Annealed Iron. 231 



Other experiments gave results of a similar character, and many 

 attempts were made to obtain an annealed wire which would not be 

 lengthened at all by magnetisation, but begin to contract from the 

 first, like nickel and cobalt. But I never succeeded in bringing a 

 wire into such a condition that its maximum elongation was less than 

 7 or 8 ten-millionths. 



Though I failed with straight wires, however, I was perfectly suc- 

 cessful with an iron ring. 



I have in a former paper given an account of some experiments 

 made with rings.* The changes in the diameter of a ring were 

 found to be of just the same character as those observed in straight 

 rods. With a continuously increasing magnetising force the diameter 

 first increased in length, and, after passing a maximum, ultimately 

 became shorter than at starting. 



In those experiments three different rings were employed, not one 

 of which had been annealed before being used ; I therefore had a new 

 ring made of good soft iron, and very thoroughly annealed. It was 

 afterwards fitted with a boxwood jacket, around which 515 turns of 

 insulated wire were wound in the usual way. With this ring it was 

 found that the smallest currents which caused any effect at all pro- 

 duced contraction ; there was no indication of the smallest prelimin- 

 ary extension. 



The results of an experiment with the ring (which I shall call 

 Ring I) are given in Table II and in the lower curve of fig. 2. 

 The latter bears a close resemblance to the earlier portion of the 

 curve given by a rod of cobalt. The greatest retraction reached 

 was nearly 75 ten-millionths (exceeding any that has been ever before 

 obtained with iron), and from the appearance of the curve there is 

 reason to believe that this was still far from its limiting value, but 

 ** the heating effect of the magnetising current made it impossible to 

 carry the experiment further. 



* ' Phil. Trans.,' 179, A, p. 205. A ring is prepared for the experiment in the 

 following manner : Short brass rods are attached to it at opposite ends of a 

 diameter, forming prolongations, which serve to transmit the effect of changes in the 

 length of the diameter to the measuring instrument. The ring is then covered 

 with a loosely fitting boxwood jacket, also ring-shaped, and through holes in this 

 the two brass rods protrude. The magnetising coil is wound directly upon the 

 wooden ring, the object of the latter being to protect the iron from the heat 

 crated when a current is passing through the coil. 



