CHANGE OF DIMENSIONS BY MAGNETIZATION. 85 



The comparison of Figs. 1 and 4 shows that the general 

 character of the change of length is the same in these two 

 cases, except that the sign of the change is opposite. Hence 

 simihir remarks as in the former hohl good in the present 

 case. 



In the experiment with nickel and cohalt wires traversed by 

 an electric cnrrent, Bidwell found that the effect was inmeasur- 

 ably small. The discrepancy in nickel perhaps arises from the 

 effect of temperature, which he did not take into account ; the 

 difference in the method adopted in the present experiment for 

 obtaining a circular field and in that of Bidwell does not seem to 

 play an important part in accounting for the said discrepancy. 

 According to the present experiment, the rise of temperature 

 occasioned a comparatively large diminution of the length change 

 in strong fields. Hence it can not be denied that in Bidwell's ex- 

 periment, the effect of circular magnetization was just as great as 

 that of temperature. The same remark will perhaps apply to his 

 experiment with cobalt ; but having no cobalt tube at my dis- 

 posal, the experimental verification must be postponed till some 

 future date. However, a theoretical deduction in favor of the 

 view above stated will be given in the last part of the paper. 



It would not be out of place to remark that a klinging 

 note of the nickel tube was heard at the make and break of 

 circular magnetizing current, a well known phenomenon. Even 

 with such a weak current as we obtain from a single Daniell's 

 cell was sufficient to produce a distinctly audible sound. 



9. It will sometimes happen that it is convenient to have 

 a simple expression for the change of length. For nickel, the 

 change of length is very well given by an empirical formula 

 of the form 



