Magnetisation in the Length of Metal Rods. 



133 



II. Steel. 



The behaviour of steel varies greatly with the hardness and temper 

 of the metal. More experiments than I have hitherto made would be 

 necessary to establish the general laws with certainty ; but my results 

 are consistent with the following conclusions : — 



1. In soft steel magnetisation produces elongation, which increases 

 up to a certain value of the magnetising force, and afterwards 

 diminishes. The maximum elongation is less than in the case of iron, 

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

 less. 



2. The critical value of the magnetising force for a steel bar 

 diminishes with increasing hardness of the steel up to a certain point 

 corresponding to a yellow temper, after which it again increases, and 

 with very hard steel becomes very high. 



3. 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. 



4. 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. 



1. xTickel continues to retract with magnetising forces far exceeding 

 those which produce the maximum elongation of iron. The greatest 

 retraction of nickel hitherto observed is more than three times as 

 great as the maximum elongation of iron, and the limit has not yet 

 been reached. 



2. A nickel wire stretched by a weight undergoes retraction when 

 magnetised. 



VOL. XL. 



L 



