1 32 Mr. S. Bidwell. On the Changes produced by 



retraction, but for reasons already referred to I was unable to measure 

 the amount. 



I have not for some weeks occupied myself with the investigation 

 of the singular facts described in this paper without from time to 

 time indulging in speculations as to their physical causes. It is, 

 however, evident enough that the investigation is incomplete, and 

 many more experiments, some of them requiring additional apparatus 

 of a special kind, remain to be tried. I hope to return to the subject 

 on a future occasion, and in the meantime refrain from theorising as 

 to the causes of the phenomena. 



Summary. 



The experiments have not been sufficiently numerous to render 

 generalisation in all cases perfectly safe ; but, so far as they go, they 

 indicate the following laws : — * 



L Iron. 



1. The length of an iron rod is increased by magnetisation up to 

 a certain critical value of the magnetising force, when a maximum 

 elongation is reached. 



2. If the critical value of the magnetising force is exceeded, the 

 elongation is diminished, until, with a sufficiently powerful force, the 

 original length of the rod is unaffected, and if the magnetising force 

 is still further increased the rod undergoes retraction. 



3. Shortly after the critical point is passed, the elongation 

 diminishes in proportion as the magnetising force increases. The 

 greatest actual retraction hitherto observed' was equal to about half 

 the greatest elongation ; but there was no indication of a limit, anc? 

 a stronger magnetising force would have produced further retrac- 

 tion. 



4. The value of the external magnetising force corresponding to 

 maximum elongation is nearly equal to twice its value at the " turning 

 point." 



Definition. — The turning point in the magnetisation of an iron bar 

 is reached when the temporary moment begins to increase less rapidly 

 than the external magnetising force. 



5. The external magnetising force corresponding to the point of 

 maximum elongation is greater for thick rods than for thin rods. 



6. The amount of the maximum elongation varies inversely as the 

 square root of the diameter of the rod. 



7. The turning point, and therefore presumably the point of maxi- 

 mum elongation, occurs with a smaller magnetising force when the 

 rod is stretched than when it is unstretched. 



The elongations and magnetisations referred to are temporary only. 



