268 Magnetisation and Lengtli of Rods of Iron, fyc. [Apr. 23, 



still further increased the rod undergoes retraction. 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 maximum elongation, 

 but there was no indication of a limit, and a stronger magnetising 

 force would have produced further retraction. 



3. The value of the external magnetising force corresponding to 

 maximum elongation is for a given rod, approximately 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. 



4. The external force corresponding to the point of maximum elon- 

 gation increases (when the quality of the iron is the same) with the 

 diameter of the rod. So also does its value at the turning point. 



5. The amount of the maximum elongation appears to vary in- 

 versely as the square root of the diameter of the rod, when the quality 

 of the iron is the same. 



6. The turning point, and therefore presumably the point of max- 

 imum elongation, occurs with a smaller magnetising force when the 

 rod is stretched than when it is unstretched. 



II. Steel. 



7. In soft steel magnetisation produces elongation, which, as in the 

 case of iron, increases up to a certain value of the magnetising force, 

 and afterwards diminishes. The maximum elongation is less than in 

 iron, and the rate of diminution after the maximum is passed is also 

 less. 



8. The critical value of the magnetising force for a steel rod dimin- 

 ishes with increasing hardness up to a certain point, corresponding to 

 a yellow temper ; after which it increases, and with very hard steel 

 becomes very high. There is therefore a critical degree of hardness 

 for which the critical magnetising force is a minimum ; in steel of a 

 yellow temper the value of the critical magnetising force is lower than 

 in steel which is either softer or harder. 



9. In soft steel a strong magnetising force subsequently diminished 

 may cause a greater temporary elongation than the diminished force is 

 capable of producing if applied in the first place. 



10. A temporary elongation when once produced in soft steel may 

 be maintained by a magnetising force which is itself too small to 

 originate any perceptible elongation. 



III. Nickel 



11. Nickel continues to retract with magnetising forces far ex- 



